Method and apparatus for atrial tachyarrhythmia detection

ABSTRACT

An implantable medical device is configured to determine a first atrial arrhythmia score from ventricular events sensed by a sensing circuit of an implantable medical device and determine a second atrial arrhythmia score from an intraventricular signal comprising atrial mechanical event signals attendant to atrial systole and produced by a sensor of the implantable medical device. An atrial arrhythmia is detected based on the first atrial arrhythmia score and the second atrial arrhythmia score.

TECHNICAL FIELD

This disclosure relates to an implantable medical device and method fordetecting atrial tachyarrhythmia using an intraventricular sensor signalincluding atrial mechanical event signals.

BACKGROUND

Implantable cardiac pacemakers are often placed in a subcutaneous pocketand coupled to one or more transvenous medical electrical leads carryingpacing and sensing electrodes positioned in the heart. A cardiacpacemaker implanted subcutaneously may be a single chamber pacemakercoupled to one transvenous medical lead for positioning electrodes inone heart chamber, atrial or ventricular, or a dual chamber pacemakercoupled to two intracardiac leads for positioning electrodes in both anatrial and a ventricular chamber. Multi-chamber pacemakers are alsoavailable that may be coupled to three leads, for example, forpositioning electrodes for pacing and sensing in one atrial chamber andboth the right and left ventricles.

Intracardiac pacemakers have recently been introduced that areimplantable within a ventricular chamber of a patient's heart fordelivering ventricular pacing pulses. Such a pacemaker may sense R-wavesignals attendant to intrinsic ventricular depolarizations and deliverventricular pacing pulses in the absence of sensed R-waves. While singlechamber ventricular sensing and pacing by an intracardiac ventricularpacemaker may adequately address some cardiac conditions, some patientsmay benefit from atrial and ventricular (dual chamber) sensing forproviding atrial-synchronized ventricular pacing and maintaining a morenormal heart rhythm.

Some patients having a pacemaker may experience tachyarrhythmia, whichmay originate in the ventricular chambers, e.g., ventricular tachycardia(VT) or ventricular fibrillation (VF). A fast ventricular rate, however,may originate in and be conducted from the atrial chambers to theventricles in patients with intact atrioventricular conduction. Anatrial tachyarrhythmia that originates in the upper heart chambers andis conducted to the ventricles is sometimes referred to assupraventricular tachyarrhythmia (SVT). Anti-tachycardia pacing therapy(ATP) and/or a cardioversion shock may be delivered to the ventricles totreat VT or VF but is likely ineffective in terminating an SVT. As such,discrimination of SVT from VT and VF enables proper diagnosis andtreatment of a heart rhythm condition.

SUMMARY

The techniques of this disclosure generally relate to an implantablemedical device configured to detect atrial tachyarrhythmia. Animplantable medical device operating according to the techniquesdisclosed herein may be implanted in the ventricle of a patient's heart.The device has a sensor configured to produce an intraventricular signalthat includes mechanical atrial systolic event signals. In someexamples, the sensor is a motion sensor, e.g., an accelerometer,configured to produce an intraventricular motion signal. The medicaldevice may be an intracardiac ventricular pacemaker configured to senseatrial mechanical events attendant atrial systole from the motion signaland sense ventricular events from a cardiac electrical signal and/or themotion signal for use in determining ventricular cycle lengths (VCLs).The pacemaker may detect atrial tachyarrhythmia based on an analysis ofVCL irregularity and atrial mechanical event evidence determined fromthe intraventricular signal. In some examples, detection of atrialmechanical events from an intraventricular motion signal duringirregular VCLs may be used to determine an atrial event evidence scorethat may be used to reduce the likelihood of detecting atrialfibrillation by the implantable medical device.

In one example, the disclosure provides an implantable medical deviceincluding a sensor configured to produce an intraventricular signalincluding an atrial mechanical event signal attendant to atrialmechanical systole, a sensing circuit configured to sense ventricularevents sensed by the sensing circuit, a memory configured to storeatrial arrhythmia episode data, and a control circuit coupled to thesensor, the sensing circuit, and the memory. The control circuit may beconfigured to determine a first atrial arrhythmia score from the sensedventricular events. The atrial arrhythmia score is correlated to apresence of atrial arrhythmia. The control circuit is further configuredto determine a second atrial arrhythmia score from the intraventricularsignal based on the atrial mechanical event signals, and detect anatrial arrhythmia based on the first atrial arrhythmia score and thesecond atrial arrhythmia score. The control circuit may generate anatrial arrhythmia detection notification in response to detecting theatrial arrhythmia. In some examples, the control circuit updates theatrial arrhythmia episode data stored in the memory including anindication of the atrial arrhythmia detection. The implantable medicaldevice may include a telemetry circuit configured to transmit theupdated atrial arrhythmia episode data. The updated atrial arrhythmiaepisode data may be transmitted to an external device for generating adisplay of the updated atrial arrhythmia episode data. In some examples,the implantable medical device includes a pulse generator configured toadjust or withhold a pacing therapy in response to the control circuitdetecting the atrial arrhythmia.

In another example, the disclosure provides a method includingdetermining a first atrial arrhythmia score from ventricular eventssensed by a sensing circuit of an implantable medical device. The firstatrial arrhythmia score is correlated to a presence of atrialarrhythmia. The method further includes determining a second atrialarrhythmia score from an intraventricular signal produced by a sensor ofthe implantable medical device. The intraventricular signal includes anatrial mechanical event signal attendant to atrial systole. The secondatrial arrhythmia score is based on the atrial mechanical event signals.The method includes detecting an atrial arrhythmia based on the firstatrial arrhythmia score and the second atrial arrhythmia score andupdating atrial arrhythmia episode data stored in a memory of theimplantable medical device in response to detecting the atrialarrhythmia. The update includes an indication of the detected atrialarrhythmia. The method may include transmitting the updated atrialarrhythmia episode data. In some examples, the updated atrial arrhythmiaepisode data is transmitted to an external device for generating adisplay of the atrial arrhythmia episode data. In some examples, themethod includes adjusting or withholding a ventricular pacing therapygenerated by a pulse generator of the implantable medical device inresponse to detecting the atrial arrhythmia.

In another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control circuit of an implantable medical device,cause the device to determine a first atrial arrhythmia score fromventricular events sensed by a sensing circuit of the implantablemedical device and determine a second atrial arrhythmia score from anintraventricular signal produced by a sensor of the implantable medicaldevice. The first atrial arrhythmia score is correlated to a presence ofatrial arrhythmia. The intraventricular signal includes an atrialmechanical event signal attendant to atrial systole. The second atrialarrhythmia score is based on the atrial mechanical event signals. Theinstructions further cause the implantable medical device to detect anatrial arrhythmia based on the first atrial arrhythmia score and thesecond atrial arrhythmia score and update atrial arrhythmia episode datastored in a memory of the implantable medical device in response todetecting the atrial arrhythmia. The instructions may cause the deviceto transmit the atrial arrhythmia episode data in response to theupdating, including an indication of the detected atrial arrhythmia. Insome examples the updated atrial arrhythmia episode data is transmittedto an external device for generating a display of the atrial arrhythmiaepisode data. The instructions may cause a pulse generator of theimplantable medical device to adjust or withhold a ventricular pacingtherapy in response to detecting the atrial arrhythmia.

The details of one or more aspects of the disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the techniques described in this disclosurewill be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an intracardiac pacingsystem that may be used to detect atrial tachyarrhythmia according tothe techniques disclosed herein.

FIG. 2 is a conceptual diagram of the intracardiac pacemaker shown inFIG. 1.

FIG. 3 is a schematic diagram of an example configuration of thepacemaker shown in FIG. 1.

FIG. 4 is an example of a motion signal that may be produced by a motionsensor included in the pacemaker of FIG. 1 over a cardiac cycle.

FIG. 5 is a flow chart of a method for detecting a cardiac arrhythmiathat may be performed by the pacemaker of FIG. 1 according to oneexample.

FIG. 6 is a flow chart of a method for detecting a tachyarrhythmiaaccording to another example.

FIG. 7 depicts an example of intraventricular motion sensor signalsacquired over two different cardiac cycles.

FIG. 8 is a flow chart of a method for determining an atrial arrhythmiaevidence score based on a combination of atrial events sensed from amotion signal and ventricular cycle lengths.

FIG. 9 is a flow chart of a method that may be performed for detectingan atrial arrhythmia according to another example.

FIG. 10 is a flow chart of a method for detecting an atrial arrhythmiausing an intraventricular motion sensor signal and VCLs according toanother example.

FIG. 11 is a graph of an example accelerometer signal, electrocardiogram(ECG) signal with event markers and ventricular electrogram (EGM) signalduring a non-atrial tracking pacing mode with normal AV conductionintact.

FIG. 12 is a graph of an example accelerometer signal, ECG signal withevent markers, and ventricular EGM signal during atrial fibrillation.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for detecting atrialtachyarrhythmia by an implantable medical device. In various examples,the implantable medical device is an intracardiac pacemaker whollyimplantable in a ventricular chamber. P-waves, attendant to atrialdepolarizations, are generally difficult to sense reliably from thecardiac electrical signal received by an intracardiac ventricularpacemaker. P-wave sensing may not be practical for detecting anddiscriminating atrial tachyarrhythmias from ventricular tachyarrhythmiasby an intracardiac ventricular pacemaker. Yet the detection anddiscrimination of atrial and ventricular tachyarrhythmias is importantinformation that clinicians have grown to expect to receive fromimplantable pacemakers that have have both atrial and ventricularelectrodes in place in the atrial and ventricular heart chambers,respectively, for sensing atrial and ventricular electrical signals.Detecting and discriminating atrial and ventricular arrhythmias may alsobe used by an implantable pacemaker in properly controlling therapydelivery.

Using techniques disclosed herein, a single chamber, intracardiacventricular pacemaker is configured to detect atrial tachyarrhythmiabased on an analysis of ventricular events in combination with ananalysis of atrial mechanical events sensed from an intraventricularsignal. As described below, atrial mechanical events attendant to atrialmechanical systole are sensed from an intraventricular signal producedby a sensor included in the pacemaker implantable within a ventricularchamber of the heart. For example, a motion signal produced by anaccelerometer includes an atrial systolic event signal corresponding toatrial mechanical contraction and the active filling phase of theventricle, sometimes referred to as the “atrial kick.” A sensor signalacquired from within a ventricular chamber is referred to herein as an“intraventricular signal.” An intracardiac ventricular pacemaker mayreceive a cardiac electrical signal for sensing R-waves attendant to thedepolarization of the ventricular myocardium for determining evidence ofatrial arrhythmia. For example, irregularity of VCLs is a positiveindicator for atrial fibrillation (AF). As disclosed herein, anintracardiac ventricular pacemaker may be configured to use evidence ofone or more atrial systolic mechanical events sensed from anintraventricular signal for reducing the likelihood of detecting atrialfibrillation based on evidence of VCL irregularity.

FIG. 1 is a conceptual diagram illustrating an intracardiac pacingsystem 10 that may be used to sense cardiac electrical signals andmotion signals induced by cardiac motion and flowing blood and providepacing therapy to a patient's heart 8. System 10 includes a ventricularintracardiac pacemaker 14, which may be capable of wirelesscommunication with an external device 20. Pacemaker 14 may be atranscatheter intracardiac pacemaker which is adapted for implantationwholly within a heart chamber, e.g., wholly within the right ventricle(RV) or wholly within the left ventricle (LV) of heart 8 for sensingcardiac signals and delivering ventricular pacing pulses. Pacemaker 14may be reduced in size compared to subcutaneously implanted pacemakersand may be generally cylindrical in shape to enable transvenousimplantation via a delivery catheter.

Pacemaker 14 is shown positioned in the RV, along an endocardial wall,e.g., near the RV apex though other locations are possible. Thetechniques disclosed herein are not limited to the pacemaker locationshown in the example of FIG. 1 and other positions within heart 8 arepossible. For example, ventricular intracardiac pacemaker 14 may bepositioned in the LV and configured to detect cardiac motion signals anddetermine ventricular cycle lengths for detecting atrial tachyarrhythmiausing the techniques disclosed herein. Pacemaker 14 may be positionedwithin the RV or LV to provide respective right ventricular or leftventricular pacing, sensing cardiac electrical signals and sensingcardiac motion signals by a motion sensor within the ventricularchamber.

Pacemaker 14 is capable of producing electrical stimulation pulses,e.g., pacing pulses, delivered to heart 8 via one or more electrodes onthe outer housing of the pacemaker. For example, in the position shown,pacemaker 14 is configured to deliver RV pacing pulses and sense an RVcardiac electrical signal using housing based electrodes for producingan RV electrogram (EGM) signal. The cardiac electrical signals may besensed using the housing-based electrodes that are also used to deliverpacing pulses to the RV.

Pacemaker 14 may be configured to control ventricular pacing in a mannerthat promotes synchrony between atrial activation and ventricularactivation, e.g., by maintaining a target atrioventricular (AV) intervalbetween atrial and ventricular pacing pulses delivered by pacemaker 14.That is, pacemaker 14 controls ventricular pacing pulse delivery tomaintain a desired AV interval between atrial contractions correspondingto atrial systole and ventricular pacing pulses delivered to causeventricular depolarization followed by ventricular contraction,corresponding to ventricular systole. At times, pacemaker 14 may beconfigured to control the delivery of ventricular pacing pulses to theRV to maintain a desired minimum ventricular rate, e.g., in a VVI orVVIR pacing mode, that is asynchronous with atrial events. The minimumventricular rate may be a programmed lower rate, e.g., 40 to 60 pulsesper minute, or a temporary lower rate set based on patient activity asan indication of metabolic demand. Pacemaker 14 may use the motionsignal from the motion sensor to determine a patient activity metric forcontrolling the ventricular pacing rate to meet the patient's metabolicdemand, as indicated by the patient activity metric.

Atrial systolic events producing the active ventricular filling phaseare detected by pacemaker 14 from a motion sensor such as anaccelerometer enclosed by the housing of pacemaker 14. The motion signalproduced by an accelerometer implanted within a ventricular chamberincludes motion signals caused by ventricular and atrial events. Forexample, acceleration of blood flowing into the RV through the tricuspidvalve 16 between the RA and RV caused by atrial systole, or the “atrialkick,” may be detected by pacemaker 14 from the signal produced by anaccelerometer included in pacemaker 14. Other cardiac signals that maybe detected by pacemaker 14 from an intraventricular motion signal, suchas cardiac motion signals caused by ventricular contraction andrelaxation, are described below, e.g., in conjunction with FIG. 4.

Atrial P-waves that are attendant to atrial depolarization arerelatively low amplitude signals in the near-field ventricular cardiacelectrical signal received by pacemaker 14 (e.g., compared to thenear-field R-wave) and therefore can be difficult to reliably detectfrom the cardiac electrical signal acquired by pacemaker 14 implanted ina ventricular chamber. Detecting an atrial rhythm for tachyarrhythmiamonitoring and/or atrial-synchronized ventricular pacing by pacemaker 14may not be reliable when based solely on a cardiac electrical signalreceived by pacemaker 14. According to the techniques disclosed herein,pacemaker 14 includes a motion sensor, such as an accelerometer, and isconfigured to detect an atrial event corresponding to atrial mechanicalactivation, or atrial systole, from a signal produced by the motionsensor. Ventricular pacing pulses may be synchronized to the atrialevent that is detected from the motion sensor signal by setting aprogrammable AV pacing interval that controls the timing of theventricular pacing pulse relative to the detected atrial systolic event.As described below, atrial event detection from the motion signal may beused for detecting atrial tachyarrhythmia that is additionally based onan analysis of VCLs determined as the time intervals between successiveventricular events, e.g., sense R-waves.

Pacemaker 14 may be capable of bidirectional wireless communication withan external device 20 for programming cardiac electrical signal sensingand pacing control parameters as well as mechanical event (motionsignal) sensing parameters, which may be utilized for detectingventricular mechanical events and the atrial systolic event from themotion sensor signal. Aspects of external device 20 may generallycorrespond to the external programming/monitoring unit disclosed in U.S.Pat. No. 5,507,782 (Kieval, et al.), hereby incorporated herein byreference in its entirety. External device 20 is often referred to as a“programmer” because it is typically used by a physician, technician,nurse, clinician or other qualified user for programming operatingparameters in pacemaker 14. External device 20 may be located in aclinic, hospital or other medical facility. External device 20 mayalternatively be embodied as a home monitor or a handheld device thatmay be used in a medical facility, in the patient's home, or anotherlocation. Operating parameters, including sensing and therapy deliverycontrol parameters, may be programmed into pacemaker 14 using externaldevice 20.

External device 20 is configured for bidirectional communication withimplantable telemetry circuitry included in pacemaker 14. Externaldevice 20 establishes a wireless communication link 24 with pacemaker14. Communication link 24 may be established using a radio frequency(RF) link such as BLUETOOTH®, Wi-Fi, Medical Implant CommunicationService (MICS) or other communication bandwidth. In some examples,external device 20 may include a programming head that is placedproximate pacemaker 14 to establish and maintain a communication link24, and in other examples external device 20 and pacemaker 14 may beconfigured to communicate using a distance telemetry and circuitry thatdoes not require the use of a programming head and does not require userintervention to maintain a communication link. An example RF telemetrycommunication system that may be implemented in system 10 is generallydisclosed in U.S. Pat. No. 5,683,432 (Goedeke, et al.), herebyincorporated herein by reference in its entirety. External device 20 maydisplay data and information relating to pacemaker functions to a userfor reviewing pacemaker operation and programmed parameters as well asEGM signals transmitted from pacemaker 14, motion sensor signalsacquired by pacemaker 14, or other physiological data that is acquiredby and retrieved from pacemaker 14 during an interrogation session. Datatransmitted to external device 20 from pacemaker 14 may include themotion sensor signal, the cardiac electrical signal, and/or a markerchannel indicating the timing and intervals of events sensed from themotion sensor signal and/or an episode of the cardiac electrical signalassociated with a tachyarrhythmia detection.

It is contemplated that external device 20 may be in wired or wirelessconnection to a communications network via a telemetry circuit thatincludes a transceiver and antenna or via a hardwired communication linefor transferring data to a centralized database or computer to allowremote management of the patient. Remote patient management systemsincluding a centralized patient database may be configured to utilizethe presently disclosed techniques to enable a clinician to review EGM,motion sensor signal, and marker channel data and authorize programmingof sensing and therapy control parameters in pacemaker 14, e.g., afterviewing a visual representation of EGM, motion sensor signal and markerchannel data.

FIG. 2 is a conceptual diagram of the intracardiac pacemaker 14 shown inFIG. 1. Pacemaker 14 includes electrodes 162 and 164 spaced apart alongthe housing 150 of pacemaker 14 for sensing cardiac electrical signalsand delivering pacing pulses. Electrode 164 is shown as a tip electrodeextending from a distal end 102 of pacemaker 14, and electrode 162 isshown as a ring electrode along a mid-portion of housing 150, forexample adjacent proximal end 104. Distal end 102 is referred to as“distal” in that it is expected to be the leading end as pacemaker 14 isadvanced through a delivery tool, such as a catheter, and placed againsta targeted pacing site.

Electrodes 162 and 164 form an anode and cathode pair for bipolarcardiac pacing and sensing. In alternative embodiments, pacemaker 14 mayinclude two or more ring electrodes, two tip electrodes, and/or othertypes of electrodes exposed along pacemaker housing 150 for deliveringelectrical stimulation to heart 8 and sensing cardiac electricalsignals. Electrodes 162 and 164 may be, without limitation, titanium,platinum, iridium or alloys thereof and may include a low polarizingcoating, such as titanium nitride, iridium oxide, ruthenium oxide,platinum black among others. Electrodes 162 and 164 may be positioned atlocations along pacemaker 14 other than the locations shown.

Housing 150 is formed from a biocompatible material, such as a stainlesssteel or titanium alloy. In some examples, the housing 150 may includean insulating coating. Examples of insulating coatings include parylene,urethane, PEEK, or polyimide among others. The entirety of the housing150 may be insulated, but only electrodes 162 and 164 uninsulated.Electrode 164 may serve as a cathode electrode and be coupled tointernal circuitry enclosed by housing 150, e.g., a pacing pulsegenerator and cardiac electrical signal sensing circuitry, via anelectrical feedthrough crossing housing 150. Electrode 162 may be formedas a conductive portion of housing 150 defining a ring electrode that iselectrically isolated from the other portions of the housing 150 asgenerally shown in FIG. 2. In other examples, the entire periphery ofthe housing 150 may function as an electrode that is electricallyisolated from tip electrode 164, instead of providing a localized ringelectrode such as anode electrode 162. Electrode 162 formed along anelectrically conductive portion of housing 150 serves as a return anodeduring pacing and sensing.

The housing 150 includes a control electronics subassembly 152, whichhouses the electronics for sensing cardiac signals, producing pacingpulses and controlling therapy delivery and other functions of pacemaker14 as described below in conjunction with FIG. 3. A motion sensor may beimplemented as an accelerometer enclosed within housing 150 in someexamples. The accelerometer provides a signal to a processor included incontrol electronics subassembly 152 for signal processing and analysisfor detecting atrial systolic events for use in tachyarrhythmiadetection and for timing ventricular pacing pulses in some examples.

The accelerometer may be a three-dimensional accelerometer having one“longitudinal” axis that is parallel to or aligned with the longitudinalaxis 108 of pacemaker 14 and two, orthogonal axes that extend in radialdirections relative to the longitudinal axis 108. Practice of thetechniques disclosed herein, however, are not limited to a particularorientation of the accelerometer within or along housing 150. In otherexamples, a one-dimensional accelerometer may be used to obtain anintracardiac motion signal from which atrial systolic events aredetected. In still other examples, a two dimensional accelerometer orother multi-dimensional accelerometer may be used. Each axis of a singleor multi-dimensional accelerometer may be defined by a piezoelectricelement, micro-electrical mechanical system (MEMS) device or othersensor element capable of producing an electrical signal in response tochanges in acceleration imparted on the sensor element, e.g., byconverting the acceleration to a force or displacement that is convertedto the electrical signal. In a multi-dimensional accelerometer, thesensor elements may be arranged orthogonally with each sensor elementaxis orthogonal relative to the other sensor element axes. Orthogonalarrangement of the elements of a multi-axis accelerometer, however, isnot necessarily required.

Each sensor element may produce an acceleration signal corresponding toa vector aligned with the axis of the sensor element. One, two or allthree vector signals produced by a three dimensional accelerometer maybe selected for use in detecting atrial systolic events for use indetecting and discriminating tachyarrhythmia and controllingatrial-synchronized ventricular pacing delivered by pacemaker 14.

Housing 150 further includes a battery subassembly 160, which providespower to the control electronics subassembly 152. Battery subassembly160 may include features of the batteries disclosed in commonly-assignedU.S. Pat. No. 8,433,409 (Johnson, et al.) and U.S. Pat. No. 8,541,131(Lund, et al.), both of which are hereby incorporated by referenceherein in their entirety.

Pacemaker 14 may include a set of fixation tines 166 to secure pacemaker14 to patient tissue, e.g., by actively engaging with the ventricularendocardium and/or interacting with the ventricular trabeculae. Fixationtines 166 are configured to anchor pacemaker 14 to position electrode164 in operative proximity to a targeted tissue for deliveringtherapeutic electrical stimulation pulses. Numerous types of activeand/or passive fixation members may be employed for anchoring orstabilizing pacemaker 14 in an implant position. Pacemaker 14 mayinclude a set of fixation tines as disclosed in commonly-assigned U.S.Pat. No. 9,775,872 (Grubac, et al.), hereby incorporated herein byreference in its entirety.

Pacemaker 14 may optionally include a delivery tool interface 158.Delivery tool interface 158 may be located at the proximal end 104 ofpacemaker 14 and is configured to connect to a delivery device, such asa catheter, used to position pacemaker 14 at an implant location duringan implantation procedure, for example within a heart chamber.

FIG. 3 is a schematic diagram of an example configuration of pacemaker14 shown in FIG. 1. Pacemaker 14 includes a pulse generator 202, acardiac electrical signal sensing circuit 204, a control circuit 206,memory 210, telemetry circuit 208, sensor 212 and a power source 214.Sensor 212 is configured to produce an intraventricular signal includingatrial mechanical event signals attendant to atrial mechanical systole.The illustrative examples described in conjunction with the accompanyingdrawings primarily refer to an intraventricular signal produced by amotion sensor. Sensor 212, therefore, may also be referred to herein as“motion sensor 212.” The techniques disclosed herein, however, may beimplemented in conjunction with other sensors capable of generating anintraventricular signal including atrial mechanical event signals.Examples of sensors that may be included in sensor 212 for sensing themechanical atrial systolic events for use in determining an atrialarrhythmia score include motion sensors, such as piezoelectric sensorsor MEMS devices, an impedance sensor capable of injecting an impedancedrive current (or voltage) signal and sensing the resultantintraventricular voltage (or current) signal, or a pressure sensor forsensing an intraventricular pressure signal, as examples.

When sensor 212 is includes an impedance sensor, an atrial systolicevent signal attendant to the mechanical atrial contraction or “atrialkick” may be detected from an intraventricular impedance signal usingtechniques for detecting the A-wave portion of an impedance signal asgenerally disclosed in U.S. Pat. No. 8,639,328 (Hettrick, et al.),incorporated herein by reference in its entirety. When sensor 212includes a pressure sensor, an atrial systolic event signal attendant tothe mechanical atrial contraction may be detected from theintraventricular pressure signal using techniques generally disclosed inU.S. Publication. No. 2011/0160787 (Greenhut, et al.). The atrialsystolic events sensed from the intraventricular sensor signal may beused for determining an atrial event evidence score for use in atrialtachyarrhythmia detection as described below.

In some examples, sensor 212 may include a multi-axis accelerometer,e.g., a two-dimensional or three-dimensional accelerometer, with eachaxis providing a signal that may be analyzed individually or incombination for detecting cardiac mechanical events. Motion sensor 212produces an electrical signal correlated to motion or vibration ofsensor 212 (and pacemaker 14), e.g., when subjected to flowing blood,cardiac motion and patient physical activity. The motion sensor 212 mayinclude one or more filter, amplifier, rectifier, analog-to-digitalconverter (ADC) and/or other components for producing a motion signalthat is passed to control circuit 206. For example, each vector signalcorresponding to each individual axis of a multi-axis accelerometer maybe filtered by a high pass filter, e.g., a 10 Hz high pass filter,digitized by an ADC and rectified for use by atrial event detectorcircuit 240 for detecting atrial systolic events. The high pass filtermay have a higher or lower cutoff frequency, e.g., 5 Hz, if needed todetect atrial signals that have higher or lower frequency content inother examples. In some examples, high pass filtering is performed withno low pass filtering. In other examples, each accelerometer axis signalis filtered by a low pass filter, e.g., a 30 Hz low pass filter, with orwithout high pass filtering.

One example of an accelerometer for use in implantable medical devicesthat may be included in motion sensor 212 is generally disclosed in U.S.Pat. No. 5,885,471 (Ruben, et al.), incorporated herein by reference inits entirety. An implantable medical device arrangement including apiezoelectric accelerometer for detecting patient motion is disclosed,for example, in U.S. Pat. No. 4,485,813 (Anderson, et al.) and U.S. Pat.No. 5,052,388 (Sivula, et al.), both of which patents are herebyincorporated by reference herein in their entirety. Examples ofthree-dimensional accelerometers that may be implemented in pacemaker 14and used for detecting cardiac mechanical events using the presentlydisclosed techniques are generally described in U.S. Pat. No. 5,593,431(Sheldon) and U.S. Pat. No. 6,044,297 (Sheldon), both of which areincorporated herein by reference in their entirety. Other accelerometerdesigns may be used for producing an electrical signal that iscorrelated to motion imparted on pacemaker 14 due to ventricular andatrial events.

The various circuits represented in FIG. 3 may be combined on one ormore integrated circuit boards which include a specific integratedcircuit (ASIC), an electronic circuit, a processor (shared, dedicated,or group) and memory that execute one or more software or firmwareprograms, a combinational logic circuit, state machine or other suitablecomponents that provide the described functionality.

Sensing circuit 204 is configured to receive a cardiac electrical signalvia electrodes 162 and 164 by a pre-filter and amplifier circuit 220.Pre-filter and amplifier circuit may include a high pass filter toremove DC offset, e.g., a 2.5 to 5 Hz high pass filter, or a widebandfilter having a passband of 2.5 Hz to 100 Hz to remove DC offset andhigh frequency noise. Pre-filter and amplifier circuit 220 may furtherinclude an amplifier to amplify the “raw” cardiac electrical signalpassed to analog-to-digital converter (ADC) 226. ADC 226 may pass amulti-bit, digital electrogram (EGM) signal to control circuit 206 foruse by atrial event detector circuit 240 and/or processor 244 inidentifying ventricular electrical events (e.g., R-waves or T-waves)and/or atrial electrical events, e.g., P-waves. Identification ofcardiac electrical events may be used for detecting atrial systolicevents from the motion sensor signal and in determining cardiac eventintervals for use in detecting and discriminating ventriculartachyarrhythmia from SVT. The digital signal from ADC 226 may be passedto rectifier and amplifier circuit 222, which may include a rectifier,bandpass filter, and amplifier for passing a cardiac signal to R-wavedetector 224.

R-wave detector 224 may include a sense amplifier or other detectioncircuitry that compares the incoming rectified, cardiac electricalsignal to an R-wave sensing threshold, which may be an auto-adjustingthreshold. When the incoming signal crosses the R-wave sensingthreshold, the R-wave detector 224 produces an R-wave sensed eventsignal (R-sense) that is passed to control circuit 206. In otherexamples, R-wave detector 224 may receive the digital output of ADC 226for detecting R-waves by a comparator, morphological signal analysis ofthe digital EGM signal or other R-wave detection techniques. Processor244 may provide sensing control signals to sensing circuit 204, e.g.,R-wave sensing threshold, sensitivity, and various blanking andrefractory intervals applied to the cardiac electrical signal forcontrolling R-wave sensing. R-wave sensed event signals passed fromR-wave detector 224 to control circuit 206 may be used for schedulingventricular pacing pulses by pace timing circuit 242 and for determiningVCLs for use in atrial tachyarrhythmia detection as described below. TheR-wave sensed event signals received from R-wave detector 224 may alsobe used by atrial event detector circuit 240 in detecting atrialsystolic events from a signal received from motion sensor 212. Forexample, R-wave sensed event signals may be used in setting one or moretiming windows to facilitate atrial event sensing from the motion signaland to determine of how many atrial events are sensed during a singleVCL.

Control circuit 206 includes an atrial event detector circuit 240, pacetiming circuit 242, and processor 244. Atrial event detector circuit 240is configured to detect atrial mechanical events, and in particularatrial systolic events, from a signal received from motion sensor 212.In some examples, one or more ventricular mechanical events may bedetected from the motion sensor signal in a given cardiac cycle tofacilitate positive detection of the atrial systolic event from themotion sensor signal during the ventricular cycle.

Control circuit 206 may receive R-wave sensed event signals and/ordigital cardiac electrical signals from sensing circuit 204 for use indetecting and confirming cardiac events and controlling ventricularpacing. R-wave sensed event signals may be passed to pace timing circuit242 for inhibiting scheduled ventricular pacing pulses or schedulingventricular pacing pulses when pacemaker 14 is operating in a non-atrialtracking ventricular pacing mode. Pace timing circuit 242 may determinetime intervals between consecutively received R-wave sensed eventsignals, referred to herein as RR intervals or RRIs, for use byprocessor 244 in a tachyarrhythmia detection. As described below,differences between consecutive ventricular cycle lengths (VCLs) may bedetermined for use in determining a metric of VCL irregularity asevidence of AF. A VCL may be an RRI, the time interval between aventricular pacing pulse and an R-wave sensed event signal, or the timeinterval between two consecutive ventricular pacing pulses.

R-wave sensed event signals may be passed to atrial event detectorcircuit 240 for use in setting time windows used by control circuit 206in detecting atrial systolic events from the motion sensor signal.Atrial event detector circuit 240 receives a motion signal from motionsensor 212 and may start an atrial blanking period in response to aventricular electrical event, e.g., an R-wave sensed event signal fromsensing circuit 204 or delivery of a pacing pulse by pulse generator202. The blanking period may correspond to a time period after theventricular electrical event during which ventricular mechanical events,e.g., corresponding to ventricular contraction and isovolumic relaxationare expected to occur. Motion signal peaks that occur during the atrialblanking period are not sensed as atrial events to avoid falsely sensinga ventricular motion signal event as the atrial systolic event.

Atrial event detector circuit 240 determines if the motion sensor signalsatisfies atrial mechanical event detection criteria outside of theatrial blanking period. The motion sensor signal during the blankingperiod may be monitored by atrial event detector circuit 240 and/orprocessor 244 for the purposes of detecting ventricular mechanicalevents, which may be used for confirming or validating atrial systolicevent detection or detecting ventricular event intervals in someexamples. As such, ventricular mechanical event detection windows may beset during the atrial blanking period and may be set according topredetermined time intervals following identification of a ventricularelectrical event. Control circuit 206 may be configured to detect one ormore ventricular mechanical events during respective ventricular eventdetection windows during the atrial blanking period. The timing anddetection of the ventricular mechanical events may be used to update theatrial blanking period and/or may be used to confirm detection of theatrial event occurring subsequent to expected ventricular mechanicalevents.

Atrial event detector circuit 240 may set time windows corresponding tothe passive ventricular filling phase and the active ventricular fillingphase based on the timing of a preceding ventricular electrical event,either an R-wave sensed event signal or a ventricular pacing pulse. Amotion sensor signal crossing of an atrial event sensing thresholdduring either of these windows may be detected as the atrial systolicevent. As described below, two different atrial event sensing thresholdvalues may be established for applying during the passive filling phasewindow and after the passive filling phase window (during an activefilling phase window).

Atrial event detector circuit 240 passes an atrial event detectionsignal to processor 244 and/or pace timing circuit 242 in response todetecting an atrial event. Processor 244 may include one or more clocksfor generating clock signals that are used by pace timing circuit 242 totime out an AV pacing interval that is started upon receipt of an atrialevent detection signal from atrial event detector circuit 240. Pacetiming circuit 242 may include one or more pacing escape interval timersor counters that are used to time out the AV pacing interval, which maybe a programmable interval stored in memory 210 and retrieved byprocessor 244 for use in setting the AV pacing interval used by pacetiming circuit 242.

Pace timing circuit 242 may additionally include a lower pacing rateinterval timer for controlling a minimum ventricular pacing rate. Forexample, if an atrial systolic event is not detected from the motionsensor signal triggering a ventricular pacing pulse at the programmed AVpacing interval, a ventricular pacing pulse may be delivered by pulsegenerator 202 upon expiration of the lower pacing rate interval toprevent ventricular asystole and maintain a minimum ventricular rate. Attimes, control circuit 206 may control pulse generator 202 in anon-atrial tracking ventricular pacing mode (also referred to as“asynchronous ventricular pacing”). The non-atrial tracking ventricularpacing mode may be denoted as a VDI pacing mode in which ventricularpacing pulses are delivered in the absence of a sensed R-wave andinhibited in response to an R-wave sensed event signal from sensingcircuit 204. Dual chamber sensing may be performed during the non-atrialtracking ventricular pacing mode by sensing ventricular electricalevents by sensing circuit 204 and sensing atrial events by atrial eventdetector circuit 240 receiving a motion signal from motion sensor 212.

Processor 244 may receive atrial sensed event signals from atrial eventdetector circuit 240 and R-wave sensed event signals from sensingcircuit 204 for use in tachyarrhythmia detection. Processor 244 maydetermine differences between consecutive VCLs for determining VCLirregularity. In some examples, two consecutively determined VCLdifferences define an ordered pair of (x, y) coordinates of a pointplotted in a two dimensional Lorenz plot. The clustering anddistribution of Lorenz plot points determined from a series of VCLdifferences may be analyzed to determine a metric or score of VCLvariability as evidence for an atrial tachyarrhythmia, e.g., AF. Themetric or score may be described herein as an atrial arrhythmia score.In general, an atrial arrhythmia score provides an indication of atrialarrhythmia and is correlated to the presence of atrial arrhythmia andmay be based on VCL irregularity.

In some examples, an AF evidence score (also referred to herein as anatrial arrhythmia score) may be determined from VCL differences asgenerally disclosed in U.S. Pat. No. 7,627,368 (Houben, et al), U.S.Pat. No. 7,031,765 (Ritscher, et al.), U.S. Pat. No. 7,537,569 (Sarkar,et al.) and U.S. Pat. No. 7,623,911 (Sarkar, et al.) as examples, all ofwhich are incorporated herein by reference in their entirety. Inresponse to receiving detected atrial mechanical event signals fromatrial event detector circuit 240, processor 244 may determine a secondatrial arrhythmia score, also referred to herein as an atrial eventscore, which may be evidence against an atrial arrhythmia becausedistinct atrial mechanical event signals attendant to atrial systole areexpected to occur at a very low frequency or be non-existent during AF.The second atrial arrhythmia score, or atrial event score, may be usedto adjust the AF evidence score to decrease the likelihood of detectingAF during a period of irregular VCLs. In some examples, absence of anatrial mechanical event signal is evidence supporting an AF detectionand may be used to increase the likelihood of detecting AF, for exampleby reducing the criteria or requirements for detecting AF based on theAF evidence score. Accordingly, the second atrial arrhythmia score maybe inversely correlated to a presence of atrial arrhythmia in someexamples.

Pulse generator 202 generates electrical pacing pulses that aredelivered to the RV of the patient's heart via cathode electrode 164 andreturn anode electrode 162. In addition to providing control signals topace timing circuit 242 and pulse generator 202 for controlling thetiming of ventricular pacing pulses, processor 244 may retrieveprogrammable pacing control parameters, such as pacing pulse amplitudeand pacing pulse width, which are passed to pulse generator 202 forcontrolling pacing pulse delivery. Pulse generator 202 may includecharging circuit 230, switching circuit 232 and an output circuit 234.

Charging circuit 230 may include a holding capacitor that may be chargedto a pacing pulse amplitude by a multiple of the battery voltage signalof power source 214 under the control of a voltage regulator. The pacingpulse amplitude may be set based on a control signal from controlcircuit 206. Switching circuit 232 may control when the holdingcapacitor of charging circuit 230 is coupled to the output circuit 234for delivering the pacing pulse. For example, switching circuit 232 mayinclude a switch that is activated by a timing signal received from pacetiming circuit 242 upon expiration of an AV pacing interval (or VV lowerrate pacing interval) and kept closed for a programmed pacing pulsewidth to enable discharging of the holding capacitor of charging circuit230. The holding capacitor, previously charged to the pacing pulsevoltage amplitude, is discharged across electrodes 162 and 164 throughthe output capacitor of output circuit 234 for the programmed pacingpulse duration. Examples of pacing circuitry generally disclosed in U.S.Pat. No. 5,507,782 (Kieval, et al.) and in U.S. Pat. No. 8,532,785(Crutchfield, et al.), both of which patents are incorporated herein byreference in their entirety, may be implemented in pacemaker 14 forcharging a pacing capacitor to a predetermined pacing pulse amplitudeunder the control of control circuit 206 and delivering a pacing pulse.

Control circuit 206 may be configured to control pulse generator 202 todeliver ventricular pacing pulses according to an atrial-trackingventricular pacing mode, sometimes referred to as a VDD mode, oraccording to a non-atrial tracking ventricular pacing mode, e.g., VDI(R)or VVI(R). Control circuit 206 may control pacing mode switching betweenan atrial-tracking and a non-atrial tracking pacing mode. For example,pacemaker 14 may switch from an atrial-tracking ventricular pacing modeto a non-atrial tracking ventricular pacing mode in response todetecting high patient activity or loss of atrial event sensing. Methodsfor controlling pacing mode switching in an intracardiac ventricularpacemaker having a motion sensor are generally disclosed in U.S. Pat.No. 9,399,140 (Cho, et al) and in U.S. patent application Ser. No.15/366,933 (Demmer, et al.), both of which are incorporated herein byreference in their entirety.

In some examples, pacemaker 14 may be configured to deliveranti-tachycardia pacing (ATP) in response to detecting VT. VT may bedetected in response to a predetermined number of RRIs being within a VTinterval zone and AF detection criteria not being met. As describedherein, during a fast or irregular ventricular rate, AF detectioncriteria may be unmet when at least one atrial event is detected fromthe motion sensor signal. If VT criteria are met and AF detectioncriteria are unmet due to atrial event sensing from the motion signal,control circuit 206 may control pulse generator 202 to deliver one ormore sequences of ATP therapy.

Memory 210 may include computer-readable instructions that, whenexecuted by control circuit 206, cause control circuit 206 to performvarious functions attributed throughout this disclosure to pacemaker 14.The computer-readable instructions may be encoded within memory 210.Memory 210 may include any non-transitory, computer-readable storagemedia including any volatile, non-volatile, magnetic, optical, orelectrical media, such as a random access memory (RAM), read-only memory(ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM(EEPROM), flash memory, or other digital media. Memory 210 may storetiming intervals and other data used by control circuit 206 to controlthe delivery of pacing pulses by pulse generator 202, e.g., by detectingan atrial event by atrial event detector circuit 240 from the motionsensor signal and setting a pacing escape interval timer included inpace timing circuit 242.

Memory 210 may be configured to store tachyarrhythmia episode data fortransmission to external device 20. In some examples, in response todetecting AF, control circuit 206 may store a segment of the ventricularEGM signal and/or motion sensor signal and/or store data relating toatrial tachyarrhythmia episode detections such as updating an atrialtachyarrhythmia or AF burden metric, storing the number and duration ofdetected atrial tachyarrhythmia episodes, or the like. Control circuit206 may store similar data associated with ventricular tachyarrhythmiaepisode detections, such as VT episode recordings of the EGM signaland/or motion signal, the number of VT episodes detected and respectiveVT episode duration as well as any delivered ATP therapy and therapyoutcome.

In some examples, control circuit 206 may be configured to generate analert or notification in response to detecting an atrial tachyarrhythmiaor when the atrial tachyarrhythmia burden reaches a threshold level,e.g. x minutes out of the most recent 24 hours. The alert may betransmitted to external device 20 via telemetry circuit 208 to notifythe patient and/or a clinician of the detected atrial tachyarrhythmia. AVT alert may be generated in response to detecting one or more VTepisodes.

Power source 214, which may be included in battery subassembly 160,provides power to each of the other circuits and components of pacemaker14 as required. Power source 214 may include one or more energy storagedevices, such as one or more rechargeable or non-rechargeable batteries.The connections between power source 214 and other pacemaker circuitsand components are not shown in FIG. 3 for the sake of clarity but areto be understood from the general block diagram of FIG. 3. For example,power source 214 may provide power as needed to charging and switchingcircuitry included in pulse generator 202; amplifiers, ADC 226 and othercomponents of sensing circuit 204; telemetry circuit 208; memory 210,and motion sensor 212.

Telemetry circuit 208 includes a transceiver 209 and antenna 211 fortransferring and receiving data, e.g., via a radio frequency (RF)communication link. Telemetry circuit 208 may be capable ofbi-directional communication with external device 20 (FIG. 1) asdescribed above. Programmable control parameters for controlling andperforming various pacemaker functions, such as atrial event detection,ventricular pacing, tachyarrhythmia detection and associated response(s)such as generating notifications or alerts or controlling therapy, maybe received by telemetry circuit 208 and stored in memory 210 for accessby control circuit 206. Intraventricular signals sensed by sensor 212,e.g., motion sensor signals, and cardiac electrical signals and/or dataderived therefrom may be transmitted by telemetry circuit 208 toexternal device 20. For example, control circuit 206 may generate anatrial arrhythmia episode notification and/or update atrial arrhythmiaepisode data stored in memory 201 and control telemetry circuit 206 totransmit the notification and/or updated atrial arrhythmia episode datato external device 20 for generating a display of the atrial arrhythmiaepisode data. The atrial arrhythmia episode data may include recordingsof intraventricular signals including an indication of sensed atrialsystolic events, cardiac electrical signals including an indication ofsensed R-waves, atrial arrhythmia episode duration, atrial arrhythmiaburden, a number of atrial arrhythmia episodes detected and/or otheratrial arrhythmia episode data. A display of the data provides usefuldiagnostic information for a clinician in selecting or managing therapyfor the patient.

The functions attributed to pacemaker 14 herein may be embodied as oneor more processors, controllers, hardware, firmware, software, or anycombination thereof. Depiction of different features as specificcircuitry is intended to highlight different functional aspects and doesnot necessarily imply that such functions must be realized by separatehardware, firmware or software components or by any particular circuitarchitecture. Rather, functionality associated with one or more circuitsdescribed herein may be performed by separate hardware, firmware orsoftware components, or integrated within common hardware, firmware orsoftware components. For example, atrial systolic event detection fromthe motion sensor signal, ventricular pacing control operations, andtachyarrhythmia detection performed by pacemaker 14 may be implementedin control circuit 206 executing instructions stored in memory 210 andrelying on input from sensing circuit 204 and motion sensor 212.Providing software, hardware, and/or firmware to accomplish thedescribed functionality in the context of any modern pacemaker, giventhe disclosure herein, is within the abilities of one of skill in theart.

FIG. 4 is an example of a motion sensor signal 250 that may be producedby motion sensor 212 over a cardiac cycle during normal sinus rhythm oratrial synchronized ventricular pacing. Vertical dashed lines 252 and262 denote the timing of two consecutive ventricular events (anintrinsic ventricular depolarization or a ventricular pace), marking therespective beginning and end of the ventricular cycle 251. The motionsignal includes an A1 event 254, an A2 event 256, an A3 event 258 and anA4 event 260. The A1 event 254 is an acceleration signal (in thisexample when motion sensor 212 is implemented as an accelerometer) thatoccurs during ventricular contraction and marks ventricular mechanicalsystole. The A1 event is also referred to herein as a “ventricularcontraction event.” The A2 event 256 is an acceleration signal that mayoccur during closure of the aortic and pulmonic valves and marks theapproximate offset or end of ventricular mechanical systole. The A2event may also mark the beginning of ventricular diastole and isgenerally an indication of the isovolumic relaxation phase of theventricles that occurs with aortic and pulmonic valve closure. The A1and A2 events may mark the start and end, respectively, of a systolictime interval.

The A3 event 258 is an acceleration signal that occurs during passiveventricular filling and marks ventricular mechanical diastole. The A3event is also referred to herein as the “A3 signal” and as the“ventricular passive filling event.” Since the A2 event occurs with theend of ventricular systole, it is an indicator of the onset ofventricular diastole. The A3 event occurs during ventricular diastole.As such, the A2 and A3 events may be collectively referred to asventricular mechanical diastolic events because they are both indicatorsof the ventricular diastolic period.

The A4 event 260 is an acceleration signal that occurs during atrialcontraction and active ventricular filling and marks atrial mechanicalsystole. The A4 event 260, also referred to herein as the “A4 signal,”is the “atrial systolic event” or merely the “atrial event” that isdetected from motion sensor signal 250. Atrial event detector circuit240 detects A4 event 260 and may pass an atrial sensed event signal toprocessor 244 for use in atrial tachyarrhythmia detection. In someexamples, the A4 event signal is used by pace timing circuit 242 totrigger a ventricular pacing pulse by starting an AV pacing interval inresponse to detecting the A4 event 260. Control circuit 206 may beconfigured to detect one or more of the A1, A2, and A3 events frommotion sensor signal 250, for at least some ventricular cardiac cycles,for use in positively detecting the A4 event 260 and setting atrialevent detection control parameters. The A1, A2 and/or A3 events may bedetected and characterized to avoid false detection of A4 events andpromote reliable A4 event detection, e.g., for promoting a highconfidence of A4 event detection during atrial tachyarrhythmia detectionand for proper timing of atrial-synchronized ventricular pacing pulses.In some examples, the A1 or A2 events may be used for determining VCLs,instead of or in addition to R-waves sensed from the cardiac electricalsignal.

FIG. 5 is a flow chart 300 of a method for detecting a cardiacarrhythmia that may be performed by pacemaker 14 according to oneexample. At block 302, control circuit 206 identifies ventricularevents. The ventricular events may include sensed R-waves, ventricularpacing pulses, or A1 or A2 events sensed from the motion signal. Controlcircuit 206 determines a first atrial tachyarrhythmia score at block 304based on the identified ventricular events. The first atrialtachyarrhythmia score may be based on VCLs determined betweenconsecutive pairs of identified ventricular events in some examples. Asdescribed below, the first atrial tachyarrhythmia score may be based onpatterns of consecutive VCL differences. The first atrialtachyarrhythmia score determined at block 304 based on VCLs may bepositively correlated with the occurrence of atrial tachyarrhythmia,e.g., irregular VCLs occurring during AF. In this case a relativelyhigher first atrial tachyarrhythmia score corresponds to an increasedprobability of the atrial tachyarrhythmia being present.

At block 306, control circuit 206 determines a second atrialtachyarrhythmia score from the motion signal received from motion sensor212. In some examples, the second atrial tachyarrhythmia score may benegatively correlated with the occurrence of an atrial tachyarrhythmia.For example the second atrial tachyarrhythmia score may be increased inresponse to detecting A4 signals from the motion signal as evidence ofatrial systolic beats, which is evidence against the rhythm being AF.For example, if the A4 signal amplitude is a consistent 0.2 g units (1 gbeing the acceleration of gravity) and then drops by more than 50% toless than 0.1 g units during AF, the A4 events will no longer bedetected. If one or more A4 events are being detected, the rhythm isunlikely to be AF.

In other examples, the second atrial tachyarrhythmia score based on themotion signal may also be positively correlated with the occurrence ofan atrial arrhythmia, in which case it may increase when an atrialtachyarrhythmia is likely to be present. For example, the second atrialarrhythmia score may be increased in response to detecting shortening ofa systolic time interval from the motion signal. As described below, thesystolic time interval may be determined from the motion signal as anA1-A2 interval extending between an A1 event 254 and an A2 event 256 (asseen in FIG. 4). Detection of a shortened systolic time interval may bepositive evidence for the detection of an atrial tachyarrhythmia, e.g.,AF. In this case, the second atrial tachyarrhythmia score may beincreased when A1-A2 interval shortening is detected and be positivelycorrelated with atrial tachyarrhythmia.

In another example, if multiple A4 events are sensed during a singleVCL, the presence of the multiple A4 events may indicate atrial flutter.The second atrial tachyarrhythmia score may be increased in response todetecting multiple A4 events within a single VCL and be positiveevidence for the detection of an atrial tachyarrhythmia, e.g., atrialflutter.

At block 308, control circuit 206 may adjust the first atrial arrhythmiascore based on the second atrial arrhythmia score. For example, thefirst atrial arrhythmia score may be decreased by the second atrialarrhythmia score when the first score is positively correlated withatrial tachyarrhythmia and the second score is negatively correlatedwith atrial tachyarrhythmia. As described below, the first atrialtachyarrhythmia score may be an AF evidence score determined from VCLsand is increased during periods of VCL irregularity, which can occurduring AF. The second atrial tachyarrhythmia score may be an atrialevent evidence score determined by counting A4 events sensed from themotion signal during the VCLs. As the atrial event evidence scoreincreases, indicating the occurrence of atrial systolic events thatwould not be detected during AF, the probability of the rhythm being AFdecreases. Thus, the second atrial arrhythmia score may be subtractedfrom the first atrial arrhythmia score at block 306.

In examples where the second atrial arrhythmia score is positivelycorrelated to the presence of atrial tachyarrhythmia, the second atrialtachyarrhythmia score may be added to the first atrial tachyarrhythmiascore at block 308. For instance if the second atrial tachyarrhythmiascore is increased in response to detecting shortening of the systolictime interval between the A1 and A2 events, or in response to detectingmultiple A4 events within a single VCL, the first atrial tachyarrhythmiascore may be increased by adding the second atrial tachyarrhythmia scoreat block 308.

At block 310, the heart rhythm may be classified as either an atrialtachyarrhythmia or a non-atrial tachyarrhythmia based on the adjustedfirst atrial arrhythmia score. Various examples are described below forclassifying the heart rhythm based on the adjusted first atrialtachyarrhythmia score and subsequently detecting an atrialtachyarrhythmia and responding to the atrial tachyarrhythmia detection.For example, the first atrial tachyarrhythmia score, adjusted by thesecond atrial tachyarrhythmia score, may be determined for one or moretime segments of the cardiac electrical signal and/or the motion signal.The adjusted first atrial tachyarrhythmia score may be compared to athreshold value for classifying the heart rhythm during the timesegment. As further described below, the control circuit 206 may adjustthe threshold value that is compared to the first atrial tachyarrhythmiascore based on an analysis of the motion signal to increase or decreasethe likelihood of detecting an atrial tachyarrhythmia based on the firstatrial tachyarrhythmia score. One or more time segments may be requiredto be classified as atrial tachyarrhythmia in order to make an atrialtachyarrhythmia detection and provide a response to the atrialtachyarrhythmia detection.

FIG. 6 is a flow chart 350 of a method for detecting tachyarrhythmiaaccording to one example. Control circuit 206 may perform the method offlow chart 350 for monitoring for atrial tachyarrhythmia on an ongoingbasis. In other examples, control circuit 206 may perform the method ofFIG. 6 periodically for re-determining the presence of a previouslydetected atrial tachyarrhythmia. The atrial tachyarrhythmia detectionmethods disclosed in conjunction with FIG. 6 and other flow charts anddiagrams presented herein may be performed when pacemaker 14 isoperating in a non-atrial tracking (asynchronous) ventricular pacingmode, e.g., a VDI pacing mode. In some examples, the atrialtachyarrhythmia detection may be performed in both atrial-tracking(synchronous) and non-atrial tracking ventricular pacing modes. Whenoperating in either an atrial synchronous or atrial asynchronous pacingmode, a pacing interlock may be implemented which prevents an atrialtachyarrhythmia detection to be made when a threshold percentage ofventricular events are paced ventricular cycles. In this way, during aVDD pacing mode, for example, when A4 events are being sensed regularlyresulting in frequent synchronized ventricular pacing greater than thepacing interlock threshold percentage, the atrial tachyarrhythmia isprecluded from being detected.

At block 352, control circuit 206 identifies a ventricular event fordetermining a ventricular cycle length (VCL) at block 354. Theventricular event may be a ventricular pacing pulse or a sensed R-wave.For example, control circuit 206 may receive an R-wave sensed eventsignal from sensing circuit 204. At block 354, the VCL between thereceived R-wave sensed event signal and the most recent precedingventricular electrical event, e.g., the most recently precedingventricular pacing pulse or R-wave sensed event signal, is determined asthe VCL.

The VCL may be determined between two intrinsic, sensed R-waves, betweenan intrinsic sensed R-wave and a ventricular pacing pulse and in somecases between two ventricular pacing pulses. The method of FIG. 6 andthe atrial tachyarrhythmia detection techniques described in conjunctionwith other flow charts and diagrams presented herein generally refer todetermination of VCL based on ventricular electrical events, e.g.,sensed, intrinsic R-waves and ventricular pacing pulses. However, it isto be understood that VCLs may be determined from mechanical eventsdetected from the motion signal received from motion sensor 212 in otherexamples. For example, a VCL may be determined as an A1-A1 interval oran A2-A2 interval (see A1 event 254 and A2 event 256 in FIG. 4).

At block 356, control circuit 206 may determine an AF evidence scorebased on VCLs acquired over a predetermined time interval or based on amost recent predetermined number of VCLs. One method for determining anAF evidence score is described briefly in conjunction with FIG. 8 belowand other example techniques for determining an AF score are disclosedin the above-incorporated U.S. Pat. No. 7,627,368 (Houben, et al), U.S.Pat. No. 7,031,765 (Ritscher, et al.), U.S. Pat. No. 7,537,569 (Sarkar,et al.) and U.S. Pat. No. 7,623,911 (Sarkar, et al.). The AF evidencescore may be determined based on a distribution of the differencesbetween consecutive VCLs. Generally, a more disperse distribution of VCLdifferences is evidence of VCL irregularity and supports a detection ofAF.

At block 358, control circuit 206 may determine if atrial events aredetected from the motion signal during the VCLs used to determine the AFevidence score. In particular, control circuit 206 may determine howmany VCLs include exactly one A4 event sensed during a single,respective VCL. Control circuit 206 updates an atrial event evidencescore at block 360 based on atrial events sensed from the motion signal.The atrial event evidence score may be a metric that indicates thenumber of atrial events that are detected with a high degree ofcertainty during the predetermined time interval or predetermined numberof VCLs. The presence of clear or well-defined atrial systolic eventsignals in the motion signal, particularly one single sensed A4 eventper VCL, is evidence against an AF rhythm since atrial fibrillation willnot produce the active atrial filling or atrial kick that results in theA4 event signal in the intra-ventricular motion sensing signal.

As the atrial event evidence score increases, the less likely that atrue AF rhythm is present. Any irregularity in VCLs that may be causingthe AF evidence score to be high may be more likely due to a ventriculartachyarrhythmia, runs of atrial or ventricular ectopy, or other abnormalheart rhythm other than AF. As such, the atrial event evidence score isused to adjust the AF evidence score at block 362. In some examples, theatrial event evidence score is subtracted from the AF evidence scoresuch that the higher the atrial event evidence score, due to acorrespondingly higher number of sensed atrial events during the VCLs,the more the AF evidence score may be decreased. In other examples, asingle sensed atrial event or a predetermined number of sensed atrialevents sensed during the VCLs used to determine the AF evidence scoremay cause control circuit 206 to adjust the AF evidence score by apredetermined decrement. In some examples, the AF evidence score isdecreased by the atrial event evidence score only when the AF evidencescore is greater than a predetermined threshold and/or the atrial eventevidence score is greater than a predetermined threshold. Other methodsmay be used for adjusting the AF evidence score based on the atrialevent evidence score for generally decreasing the likelihood ofdetecting AF when at least one atrial systolic event is detected fromthe motion signal during the VCLs used to determine the AF evidencescore.

The AF evidence score is compared to an AF detection threshold at block364. If the AF evidence score, after adjusting by the atrial eventevidence score as needed, is less than the AF evidence detectionthreshold, the process returns to block 352 to continue determining VCLsfor determining the next AF evidence score. If the AF evidence score isgreater than the AF evidence detection threshold, control circuit 206may classify the interval over which the VCLs were determined as an AFinterval at block 366. If a threshold number of AF intervals aredetected (block 368), AF is detected at block 370. The threshold numberof AF intervals required to detect AF may be one or more. When two ormore AF intervals are required to be classified as AF intervals in orderto detect AF, the AF intervals may or may not be required to beconsecutive. In some examples, the techniques for detecting AF based ontime intervals that are classified as AF (or not) as generally disclosedin U.S. Publication No. 2018/0028086 (Cao, et al.) may be used incombination with the techniques of adjusting the AF evidence score basedon analysis of the motion signal as disclosed herein. The '086publication is incorporated herein by reference in its entirety.

In other examples, rather than adjusting the AF evidence score by theatrial event evidence score, control circuit 206 may use the AF evidencescore and the atrial event evidence score in one or more logicoperations for detecting AF. For instance, in one logic operation the AFevidence score is required to be equal to or greater than an AFdetection threshold, and the atrial event evidence score is required tobe less than an atrial event threshold in order to detect AF. If eitherof these criteria is not met, AF is not detected. Multiple logicoperations may be performed using different tiered combinations of AFdetection threshold and atrial event threshold. For instance, if the AFevidence score is less than a first AF detection threshold, the atrialevent evidence score may be compared to a second lower atrial eventthreshold. If the atrial event evidence score is less than the secondlower atrial event threshold and the AF evidence score is greater than asecond, lower AF detection threshold, AF may still be detected.

Pacemaker 14 may be configured to provide an AF detection response atblock 372. The AF detection response may include generating an AFdetection notification and/or storing AF episode related data such as atime and date stamp of the AF detection, a time duration of the AFepisode, the ventricular rate during the AF episode, or other AF episoderelated data. The AF detection response may include storing a timesegment of the cardiac electrical signal and/or intraventricular motionsignal acquired during the detected AF episode. The stored signal dataand/or other AF episode data may be transmitted to an external device,e.g., external device 20, for generating a display of the atrialarrhythmia episode data for review by a clinician. In some examples,control circuit 206 may be configured to sum the time duration of all AFepisodes detected over a predetermined time period, e.g., over 24 hours,one week, one month or other time period. The cumulative time durationsof AF episodes may be stored in memory 210 as an AF burden that isupdated in response to each AF detection and may be transmitted to theexternal device 20.

In addition to, or alternatively to, updating and storing AF episoderelated data, control circuit 206 may provide an AF response at block372 by switching pacing modes. The pacemaker 14 may be operating in anatrial tracking ventricular pacing mode, e.g., VDD. In response todetecting AF, control circuit 206 may switch to a non-atrial trackingventricular pacing mode, e.g., VDI or VVI, at block 372. In someexamples, control circuit 206 may switch back to the atrial trackingpacing mode when AF is no longer being detected.

In some examples, if pacemaker 14 is configured to detect VT or VF anddeliver ATP, the AF detection response provided at block 372 may includewithholding ATP. Atrial depolarizations during AF may be irregularlyconducted to the ventricles in some patients leading to SVT. ATPdelivered to the ventricle during AF is not expected to be effective interminating the tachyarrhythmia. Accordingly, in some examples, controlcircuit 206 may be configured to withhold VT detection and/or ATPtherapy delivery at block 372 in response to AF being detected.

FIG. 7 is an example of motion sensor signals 400 and 410 acquired overtwo different cardiac cycles. A ventricular pacing pulse is delivered attime 0.0 seconds for both cardiac cycles. The top sensor signal 400 isreceived over one cardiac cycle, and the bottom sensor signal 410 isreceived over a different cardiac cycle. The two signals 400 and 410 arealigned in time at 0.0 seconds, the time of the ventricular pacing pulsedelivery. While motion signals 400 and 410 and motion signal 250 of FIG.7 are shown as raw accelerometer signals, it is recognized that controlcircuit 80 may receive a digitized filtered, amplified and rectifiedsignal from motion sensor 212 for processing and analysis for sensingatrial events for use in tachyarrhythmia detection.

The A1 events 402 and 412 of the respective motion sensor signals 400and 410, which occur during ventricular contraction, are observed to bewell-aligned in time following the ventricular pacing pulse at time 0.0seconds. Similarly, the A2 events 404 and 414 (marking the isovolumicventricular relaxation phase and end of ventricular systole and start ofventricular diastole) and the A3 events 406 and 416 (occurring duringpassive ventricular filling during ventricular diastole) arewell-aligned in time. Since the A1, A2 and A3 events are ventricularevents, occurring during ventricular contraction, isovolumic ventricularrelaxation, and passive ventricular filling, respectively, these eventsare expected to occur at relatively consistent intervals following aventricular electrical event, the ventricular pacing pulse in thisexample. The time relationship of the A1, A2 and A3 events may bedifferent following a ventricular pacing pulse compared to following asensed intrinsic R-wave. During a stable paced or intrinsic ventricularrhythm, however, the relative timing of A1, A2 and A3 events to eachother and the immediately preceding ventricular electrical event isexpected to be relatively consistent from beat-to-beat. The timeinterval 452 between the A1 and A2 events may shorten, however, in thepresence of AF. Shortening of the A1-A2 interval may occur during AF dueto changes in heart chamber filling, resulting in earlier closure of theaortic and pulmonic valves and shortening of the systolic time interval.In some examples, the A1-A2 interval 452 may be determined by controlcircuit 206 for use as positive evidence of AF in detecting an atrialarrhythmia. Detection of A1-A2 interval shortening may be used bycontrol circuit 206 for increasing the likelihood of detecting AF in thepresence of VCL irregularity.

The A4 events 408 and 418 of the first and second motion sensor signals400 and 410 respectively are not aligned in time. The A4 event occursduring atrial systole and as such the time interval of the A4 eventfollowing the immediately preceding ventricular electrical event (sensedR-wave or ventricular pacing pulse) and the preceding A1 through A3ventricular events may vary between cardiac cycles.

The timing of the A1 through A3 events relative to each other and theimmediately preceding ventricular electrical event may be used fordetermining an atrial blanking period 436 and increasing confidence inreliably detecting A4 events 408 and 418. The atrial systolic event isnot detected during the atrial blanking period 436 which extends fromthe ventricular electrical event (at time 0.0) through an estimatedonset of ventricular diastole so that the atrial blanking period 436includes both the A1 and A2 events. An A3 window 424 may be set having astarting time 420 corresponding to the end of the post-ventricularatrial blanking period 436 and an ending time 425. The ending time 425may be established based on timing of the A3 event or an expected endtime of the passive ventricular filling phase after a ventricularelectrical event.

A4 events 408 and 418 may be detected based on a multi-level A4 sensingthreshold 444. As seen by the lower motion sensor signal 410, the A4event 418 may occur earlier after the A3 window 424 due to changes inatrial rate. In some instances, as the atrial rate increases, the A4event 418 may occur within the A3 window 424. When this occurs, the A3event 416 and the A4 event 418 may fuse as passive and activeventricular filling occur together. The fused A3/A4 event may have ahigh amplitude, even greater than the amplitude of either the A3 event416 or the A4 event 418 when they occur separately. As such, in someexamples a first, higher A4 sensing threshold amplitude 446 may beestablished for detecting an early A4 signal that is fused with the A3signal during the A3 window 424. A second, lower A4 sensing thresholdamplitude 448 may be established for detecting relatively later A4signals, after the ending time 425 of the A3 window 424. The A3 windowending time 425 may also be considered a starting time of an A4 sensingwindow 450, though A4 signals may be sensed during the A3 window whenthe higher sensing threshold amplitude 446 is crossed. The earliestcrossing of the A4 sensing threshold 444 by the motion sensor signalafter the starting time 420 of the A3 window (or after the expiration ofthe atrial blanking period 436) may be detected as the atrial systolicevent.

Ventricular A1 and A2 events may be sensed during the atrial blankingperiod 436 by control circuit 206 for use in monitoring A1-A2 intervalshortening. An A1 window 420 may be set based on an expected A1 timeinterval 430 between a ventricular electrical event and the A1 event 402or 412. An A2 window 422 may be set based on an expected time interval432 from the ventricular electrical event until the A2 event 404 or 414.The timing of the A1 window 420 and A2 window 422 may adapt to changingventricular rate. An A1 sensing threshold 440 may be applied to themotion sensor signal during the A1 window 420, and an A2 sensingthreshold 442 may be applied to the motion signal during the A2 window422. The time interval between an A1 sensing threshold crossing and anA2 sensing threshold crossing may be determined by control circuit 206as the A1-A2 interval 452. In other examples, a morphology waveform orfeature of the A1 signal and A2 signal may be used in addition to, oralternatively to, the two respective threshold crossings during a settime window 420 or 422 for detecting the A1 and A2 events and forsubsequently determining A1-A2 interval shortening.

FIG. 8 is a flow chart 500 of a method for determining an AF evidencescore based on a combination of atrial events sensed from a motionsignal and VCLs. An analysis of VCLs and sensed atrial events may bedetermined over a predetermined time interval, e.g. one minute, twominutes, or other time interval, which may be less than or greater thanone minute. In one example, control circuit 206 sets a timer to twominutes at block 502. At bock 504, a ventricular event is identified.The ventricular event may be an electrical event, e.g., a sensed,intrinsic R-wave or a delivered ventricular pacing pulse. In otherexamples, the ventricular events may be identified as ventricularmechanical events (e.g., A1 events) sensed from the motion signalproduced by motion sensor 212.

At block 506, control circuit 206 determines if a predetermined numberof ventricular events (sensed R-waves, delivered ventricular pacingpulses or a combination thereof) meeting VCL criteria have beenidentified. For instance, four ventricular events may be required todetermine two consecutive VCL differences. One VCL is determined betweentwo consecutively identified ventricular events, e.g., “V1” and “V2.” Asecond VCL consecutively following the first VCL is determined betweenthe second ventricular event, “V2,” and a third ventricular event, “V3.”One VCL difference, AWL, can be determined between the first and secondVCLs (e.g., VCL1−VCL2=ΔVCL1). Thus, determination of one VCL difference,ΔVCL1, requires three consecutively identified ventricular events, V1,V2 and V3.

In some examples, two VCL differences are determined to generate anordered pair (ΔVCL1, ΔVCL2) that defines a point in a two-dimensionalLorenz plot area. The distribution of ordered pairs of the VCLdifferences may be used for determining an AF evidence score. Forexample, the process of determining an AF evidence score may includepopulating histogram bins of a two-dimensional Lorenz plot area with acount of the ordered pairs (ΔVCL1, ΔVCL2) that fall into eachtwo-dimensional histogram bin range, e.g., as generally disclosed in theabove-incorporated patents. In order to determine two consecutive VCLdifferences to generate one data point defined by the ordered pair(ΔVCL1, ΔVCL2), four ventricular events need to be identified at block506. The first three ventricular events, V1, V2 and V3, define twoconsecutive VCLs and ΔVCL1. One more ventricular event, “V4,” along withthe preceding ventricular event V3 defines a third consecutive VCL. Thedifference between the second VCL and the third VCL is determined as thesecond VCL difference, ΔVCL2, for defining the ordered pair (ΔVCL1,ΔVCL2). Thus, control circuit 206 continues to identify ventricularevents at block 504 until four ventricular events are identified (block506) for determining two consecutive VCL differences at block 508.

In some examples, additional criteria are applied to the identifiedventricular events and VCLs at block 506 before determining the VCLdifferences at block 508. For instance, a maximum number of pacingpulses, e.g., one ventricular pacing pulse out of the four identifiedventricular events, may be allowed. Additionally or alternatively, eachof the three VCLs defined by the four identified ventricular events maybe required to be greater than a predetermined VCL threshold at block506. According to one example, control circuit 206 determines whethereach of the VCLs is greater than 220 milliseconds. The intervalthreshold may be an SVT limit defined to reduce the likelihood of AFdetection based on VCLs shorter than the SVT limit, which may beevidence of a true fast VT or VF.

At block 508, VCL differences are determined from the N ventricularevents meeting VCL criteria. If the timer has not expired, VCLdifferences continue to be determined for generating ordered pairs oftwo consecutive VCL differences for populating a two-dimensional Lorenzplot histogram. In other examples, VCLs may be accumulated and analyzedaccording to other VCL- or RRI-based AF detection criteria that do notnecessarily collect the VCL differences in ordered pairs for populatinga Lorenz plot histogram. The techniques disclosed herein that utilize A4event sensing from a motion signal for detecting atrial beats asevidence against an AF detection may be used in combination with avariety of AF detection criteria.

Control circuit 206 may concurrently acquire atrial event sensing dataduring the predetermined time segment, while VCL data is being acquired.Control circuit 206 may determine if A4 signals are sensed during thepredetermined time segment. The more A4 signals being sensed as one A4event per VCL during the predetermined time segment, the less likely AFis present. As such, increasing evidence of one sensed atrial event perVCL is used to decrease the probability of AF being detected.

At block 510, control circuit 206 may apply criteria for counting an A4sensed event. To reduce the likelihood of A4 oversensing, A4 eventsensing for the purposes of tachyarrhythmia detection may be performedonly during the A4 window, e.g., the time interval between the precedingA3 window ending time and the next ventricular event. During short VCLs,the A3 and A4 events may be fused making it difficult to confidentlydiscern the A4 event or the A1 event may occur early, potentially beingoversensed as A4 events during the A4 window. Accordingly, criteria maybe applied at block 510 to the A4 window and/or VCL to promote reliableA4 event sensing for use in determining the atrial event evidence score.

For example, the A4 window 450 shown in FIG. 7, between the A3 windowending time 425 and the next ventricular event (identified at block 504)may be required to be greater than a threshold time interval in order tocount an A4 event sensed during the A4 window toward reducing theprobability of detecting AF. The A4 window may be required to be atleast 400 ms as an example. If the A4 window is less than a thresholdtime duration, the A4 window may be ignored in determining an atrialevent evidence score. In other examples, the VCL may be required to belonger than a predetermined threshold at block 510 in order for A4 eventsensing during the corresponding A4 window to be used in accumulatingatrial event evidence against detecting an AF rhythm. Criteria may beapplied to the VCL at block 510 instead of or in addition to the A4window. For example, the VCL ending with the ventricular eventidentified at block 504 may be required to be greater than a thresholdVCL interval, e.g., at least 700 ms, or more. In other examples,criteria applied at block 510 may include a requirement of a minimum VCLdifference between two consecutive VCLs in order to utilize A4 eventsensing for AF detection. For instance, the VCL may be required to begreater than a threshold interval, e.g., 700 to 800 ms, and the VCLdifference with the preceding VCL may be required to be greater than adifference threshold, e.g., 100 ms, in order for an A4 event to becounted toward reducing the probability of detecting AF. The criteriaapplied at block 510 may be optional, and control circuit 206 mayincrease the A4 count at block 514 in response to each A4 sensed eventsignal received from atrial event detector 240 that represents a singleA4 sensing threshold crossing during a VCL.

Control circuit 206 waits for the next identified ventricular event atblock 504 if the A4 interval criteria are unmet at block 510 or when anA4 event is not sensed during the A4 window at block 512. In response toan A4 event being sensed during the A4 window that meets the criteriarequired at block 510 (“yes” branch of block 512), control circuit 206may increase the value of an A4 counter at block 514. In other examples,the value of the A4 counter may be increased in response to the motionsignal crossing the A4 threshold 444 during the A3 window 424 or duringthe A4 window 450 in the current ventricular cycle.

Control circuit 206 may compare the value of the A4 counter to an A4evidence threshold at block 516. If the A4 count value has not reachedan A4 evidence threshold value (“no” branch of block 516), the processreturns to block 504 to identify the next ventricular event as long asthe predetermined time interval has not expired (“no” branch of block520). If the A4 evidence threshold is reached, control circuit 206 mayincrease the atrial event evidence score at block 518. According to oneexample, the atrial event evidence score is increased by one in responseto the A4 count reaching a value of 4. In other examples, the atrialevent evidence score may be increased by one each time the A4 countreaches a predetermined value that may be less than or greater thanfour. In still other examples, the atrial event evidence score may beset equal to the A4 count so that the A4 count is used directly as theatrial event evidence score that is used to adjust the AF evidencescore.

In some examples, additional analysis of the A4 sensed events may beperformed before increasing the atrial event evidence score at block518. For example, a predetermined number of sensed A4 event signals,with each being detected as one A4 signal per VCL, may be ensembleaveraged. Control circuit 206 may determine one or more signal featuresfrom the ensemble averaged A4 signal and compare the signal features tocriteria for confirming that the averaged signal represents true A4signals. If the A4 signal criteria are met, control circuit 206increases the atrial event evidence score at block 518.

After increasing the atrial event evidence score (block 518), if thetimer has not expired (“no” branch of block 520), control circuit 206returns to block 504 to identify the next ventricular event andcontinues to count sensed A4 events and updating the atrial eventevidence score at block 518 as needed. Upon expiration of the timer atblock 520, control circuit 206 may determine the percentage ofidentified ventricular events that were ventricular pacing pulses duringthe expired time segment. The percentage of ventricular pacing pulsesmay be determined out of all identified ventricular events or out ofonly the ventricular events used to determine VCLs after meeting VCLcriteria at block 506.

If the percentage of ventricular events that are ventricular pacingpulses is greater than a pacing limit threshold at block 522, controlcircuit 206 may withhold classifying the currently expired time segment.VCL data acquired during relatively frequent ventricular pacing may beconsidered unreliable for detecting AF based on VCLs. If the pacing modeis an atrial-tracking ventricular pacing mode, a high frequency ofpacing determined at block 522 may be indicative of a high incidence ofA4 event sensing, which is a contraindication for detecting AF.

Accordingly, if the percentage of pacing pulses is greater than thethreshold, e.g., greater than 30%, control circuit 206 may withholddetermining an AF evidence score for the currently expired time segment.The currently expired time segment may be ignored for the purpose ofdetecting AF in some examples. In some examples, a predetermined numberof time segments may be required to be classified as AF based on the AFevidence score in order to detect AF. In this case, the currentlyexpired time segment may not be classified based on a new AF evidencescore but may be given the same classification of the preceding timesegment, held for the currently expired time segment at block 524. Forinstance, if the preceding time segment was classified as AF, thecurrently expired time segment may also be classified as AF. If theprevious time segment was classified as non-AF, the currently expiredtime segment may also be classified as non-AF.

When the percentage of pacing pulses is less than the threshold at block522, control circuit 206 determines the AF evidence score at block 526based on the VCL differences determined at block 508 and adjusted by theatrial event evidence score determined at block 518. The AF evidencescore may be determined based on a distribution of the ordered pairs ofVCL differences in a two-dimensional Lorenz plot histogram. A highnumber of occupied histogram bins indicates high VCL irregularity and isevidence of AF. A high frequency of occupied histogram bins surroundingthe origin of a two-dimensional Lorenz plot coordinate system indicatesvery low VCL irregularity, which may be evidence of normal sinus rhythmor highly organized atrial tachycardia and is not an indication of AF.In one example, the AF evidence score based on VCLs may be determined bysubtracting the number of ordered pairs of VCL differences falling inhistogram bins within a defined region around the origin from the totalnumber of occupied histogram bins outside the region around the origin.In some examples, evidence of regular premature atrial contractions(PACs) may be determined from the Lorenz plot histogram based on aclustering of ordered pairs of VCL differences that is a signaturepattern of PACs. Frequent PACs may occur at regular coupling intervalsand present regular patterns of VCLs, e.g., associated with bigeminy(short-short-long RRIs) or trigeminy (short-short-short-long RRIs).Accordingly, the AF evidence score before adjustment by the atrial eventevidence score may be determined by subtracting the count of orderedpairs within the origin region and subtracting a number of counts withinpredefined segments of the Lorenz plot histogram area corresponding toPAC patterns of VCLs. Various techniques for determining an AF evidencescore based on VCLs or other VCL irregularity metrics, which may beperformed by control circuit 206, are generally described in the aboveincorporated U.S. Pat. No. 7,031,765 (Ritscher et al.); U.S. Pat. No.7,623,911 (Sarkar, et al.); U.S. Pat. No. 7,537,569 (Sarkar, et al.),and in U.S. Pat. No. 7,627,368 (Houben et al).

The AF evidence score determined based on the VCL differences may beadjusted based on the incidence of atrial event sensing. Control circuit206 may subtract the atrial event evidence score determined at block 518from the VCL-based AF evidence score to obtain the adjusted AF evidencescore at block 526. The adjusted AF evidence score is compared to an AFthreshold at block 530 for classifying the currently expired timesegment, e.g., as AF or non-AF. The AF score threshold may be selectedand optimized based on historical clinical data of selected patientpopulations or historical individual patient data, and the optimal AFscore threshold setting may vary from patient to patient. In anillustrative example, the AF score may have a possible range of 0 to100. The AF score threshold may be set between 25 and 75. If the AFscore meets or crosses an AF score threshold, the time segment overwhich the VCLs were collected, and thus the cardiac signal occurringwithin that time segment, is classified as an AF time segment. One ormore AF time segments may be required to detect an AF rhythm, e.g., asdescribed in conjunction with FIG. 6.

FIG. 9 is a flow chart 600 of a method that may be performed fordetecting AF according to another example. At block 602, control circuit206 determines an AF evidence score based on VCL differences, e.g.,using any of the techniques described above or in the incorporatedreferences. At block 604, control circuit 206 determines atrial eventevidence. Atrial event evidence may be determined by counting the numberof sensed A4 signals, which occur as one sensed A4 signal per VCL, overthe time segment that VCLs are determined for determining the AFevidence score. An atrial event evidence score may be determined basedon sensed A4 signals, e.g., by increasing the atrial event evidencescore by one after every four (or other predetermined number of) A4sensed event signals.

Control circuit 206 may determine if AF threshold adjustment criteriaare met at block 606. Control circuit 206 may compare the number ofatrial events sensed (one per VCL) over the time segment that the VCLswere acquired to a threshold number at block 606. The number of sensedA4 signals, or an atrial event evidence score based on the number ofsensed A4 signals, may be compared to the threshold. For example, if nomore than one A4 event is sensed during a two-minute time segment, theAF threshold may be adjusted, e.g., lowered, at block 608 to increasethe likelihood of classifying the heart rhythm during the predeterminedtime segment as being AF and ultimately detecting AF. In other examples,another maximum predetermined number of sensed A4 events, as low as zeroA4 events, may be defined as criteria for adjusting the AF threshold ina manner that increases the likelihood of detecting AF. Very few or noA4 events sensed during the VCLs used to determine the AF evidence scoreis positive evidence for AF.

The percentage of pacing during the time segment over which the AFevidence score is determined may also be taken into account whendetermining if criteria for adjusting the AF detection threshold aremet. For instance, if 20 to 40% of the ventricular events identified fordetermining VCL differences are pacing pulses, and less than a maximumthreshold number of A4 events are sensed, the AF detection threshold maybe decreased at block 608. The presence of a relatively high percentageof paced ventricular cycles, e.g., greater than 40%, will reduce the AFevidence score since the VCLs will be less irregular during frequentventricular pacing. To account for this reduced irregularity of VCL, theAF detection threshold may be decreased when there are very few (or no)A4 events sensed during relatively frequent pacing to still allowdetection of AF during pacing. Accordingly, criteria defined at block606 may define one or more ranges of pacing percentages and acorresponding maximum threshold number of sensed A4 events as criteriafor adjusting the AF threshold by a respective predetermined amount.

During atrial tachycardia or atrial flutter, an atrial beat may occurmore than once during the A4 window. In some examples, multiple peaks orA4 threshold amplitude crossings during the A4 window may be evidence ofan atrial tachyarrhythmia that is not AF. As such, atrial event evidencedetermination at block 604 may include determining the number of peaksduring an A4 window or determining a number of A4 sensing thresholdcrossings during an A4 window. When multiple peaks or multiple A4sensing threshold crossings occur within a single A4 window, during oneor more VCLs, criteria for reducing the AF threshold is not met at block606. The AF threshold may remain the same or even be increased to reducethe likelihood of detecting AF when multiple A4 sensing thresholdcrossings or motion signal peaks occur within a single A4 window.

At block 610, control circuit 206 may adjust the VCL-based AF evidencescore by subtracting an atrial event evidence score as described abovein conjunction with FIG. 8. The adjusted AF evidence score is comparedto the AF detection threshold at block 612, which may be the adjusted AFdetection threshold if criteria are met at block 606. In some examples,the AF detection threshold may be adjusted based on an absence of sensedA4 signals with or without adjusting the AF evidence score based onatrial event evidence.

If the AF evidence score is greater than the AF threshold at block 612,the cardiac rhythm during the time segment over which the AF evidencescore was determined is classified as AF at block 614. An AF detectionand AF detection response may be made as described in conjunction withFIG. 6. The time segment is not classified as AF at block 616 if theadjusted AF evidence score is not greater than the AF threshold. Theprocess may return to block 602 to determine an AF evidence score forthe next time segment.

In some examples, when control circuit 206 determines that multiple A4sensing threshold crossings occur within a single VCL, for one or moreVCLs, at block 604 when determining atrial event evidence, the controlcircuit 206 may detect the atrial tachyarrhythmia as being atrialflutter at block 614 rather than AF. Additional analysis may beperformed at block 604 to determine if the morphology of the multiple A4sensing threshold crossing waveforms are similar and thus represent aseries of atrial flutter events. If the morphology is varying, theclassification at block 614 may be AF. More generally, multiple A4sensing threshold crossings within a single VCL may be evidence ofatrial tachyarrhythmia, and the classification made at block 614 may beatrial tachyarrhythmia without discrimination between atrial flutter andAF.

As shown by the flow chart of FIG. 9, an analysis of the number of A4events sensed during a time segment over which VCLs are determined maybe used to adjust the VCL-based AF evidence score, an AF thresholdapplied to the AF evidence score, or both. A low incidence or absence ofsensed A4 events, or multiple A4 sensing threshold crossings per VCL,may increase the likelihood of detecting atrial tachycardia when VCLsare irregular due to a decreased AF threshold and/or little or noadjustment of the VCL-based AF evidence score. A relatively higherincidence of A4 events sensed once per VCL decreases the likelihood ofdetecting AF due to a non-adjusted or increased AF threshold and/or adecreased AF evidence score after adjusting by the atrial eventevidence.

FIG. 10 is a flow chart 700 of a method for detecting AF using a motionsensor signal and VCLs according to another example. After determiningthe VCL-based AF evidence score at block 702, control circuit 206 maydetermine atrial event evidence from the motion signal at block 704,e.g., by counting sensed A4 events as described above. In this example,shortening of the A1-A2 interval may be determined from the motionsensor signal as positive evidence for AF. At block 706, the A1 and A2events are detected from the motion signal by control circuit 206. TheA1 and A2 events may be sensed based on threshold crossings of themotion signal after an identified ventricular electrical event, e.g.,during A1 and A2 windows as shown in FIG. 7, which may be heartrate-adaptive windows. Control circuit 206 may determine the timeinterval between the sensed A1 and A2 events. A1-A2 intervals may bedetermined during each one of multiple VCLs during the predeterminedtime segment. In other examples, the A1-A2 interval may be determined atthe beginning, at the end or during VCLs selected according to asampling rate during the AF evidence score time segment over which VCLsare collected. Multiple A1-A2 intervals determined during different VCLsmay be averaged to determine a representative A1-A2 interval during thetime segment. In some examples, the A1-A2 interval is determined duringa VCL in which no A4 event is sensed or during the VCL immediatelyfollowing a VCL in which no A4 event is sensed.

Shortening of the A1-A2 interval may be a positive indicator of AF andused to adjust the AF threshold at block 708. For instance, the AFthreshold that is compared to the AF evidence score may be decreased inresponse to detecting A1-A2 shortening to increase the likelihood ofdetecting AF during periods of irregular VCLs. Control circuit 206 maydetect A1-A2 interval shortening by comparing an A1-A2 intervaldetermined over the time period that VCLs are collected to an A1-A2interval determined previously, e.g., during a time segment having lowVCL irregularity (or low AF evidence score). A period of low VCLirregularity may be identified as a time segment in which the number ofoccupied bins in a Lorenz plot histogram is less than a predeterminedthreshold. During normal sinus rhythm, the number of (ΔVCL1, ΔVCL2)points in histogram bins around the origin of a Lorenz plot is high sothat the number of occupied bins outside the origin segment less thenumber of (ΔVCL1, ΔVCL2) points within the origin segment is a lowvalue, e.g., zero or even negative. As such, a period of low VCLirregularity may be identified when the AF evidence score is zero orless. A normal or baseline A1-A2 interval may be determined during thisperiod of low VCL irregularity for comparison to an A1-A2 intervalduring an unknown rhythm. Any time period having a low number ofoccupied bins outside the origin segment, a high number of (ΔVCL1,ΔVCL2) points in the origin segment or generally a low, non-adjusted AFevidence score may be identified as a period of low VCL variabilityduring which a normal or baseline A1-A2 interval may be determined.

A1-A2 shortening may be detected at block 706 in response to a thresholddecrease in the A1-A2 interval compared to a previously determined A1-A2interval. For example, if the A1-A2 interval determined at block 706 isat least 20% shorter than an A1-A2 interval determined during a periodof low VCL irregularity (e.g., during a known non-tachyarrhythmiarhythm), A1-A2 interval shortening may be detected. In other examples,the A1-A2 interval determined at block 706 may be compared to athreshold A1-A2 interval stored in memory 210. In response to detectingA1-A2 interval shortening (“yes” branch of block 706), the AF thresholdis adjusted at block 708. The AF threshold may be decreased in order toincrease the likelihood of detecting AF based on the AF evidence scoredetermined from the VCLs. The AF threshold may be decreased by apredetermined decrement or percentage, e.g., 20%, 30% or other value.

When A1-A2 interval shortening is not detected, the process may advancefrom block 706 to block 710 without adjusting the AF threshold. In otherexamples, the AF threshold may be adjusted at block 708 based on theatrial event evidence determined at block 704, e.g., as described abovein conjunction with FIG. 9, even though A1-A2 interval shortening is notdetected. As such, the AF threshold may be decreased at block 708 basedon the absence (or very low count) of sensed A4 events during the timesegment and/or based on detecting A1-A2 interval shortening. In variousexamples, the AF threshold may be decreased in response to either anabsence of A4 sensed events or A1-A2 interval shortening; decreased onlyif both A1-A2 interval shortening is detected and A4 events are notsensed; or decreased by a first adjustment based on an absence or lowincidence of A4 events sensed once per VCL and decreased by a secondadjustment in response to A1-A2 interval shortening.

After adjusting the AF threshold as needed at block 708, the AF evidencescore may be adjusted based on the detection of A1-A2 intervalshortening in addition to or instead of adjusting the AF threshold. Forinstance, the AF evidence score may be increased in response todetecting A1-A2 interval shortening. The increase may be a predeterminedincrement or percentage of the AF evidence score, making it more likelythat AF is detected during a period of VCL irregularity. The AF evidencescore may additionally or alternatively be adjusted based on the atrialevent evidence as described above.

At block 712, the AF evidence score is compared to the AF threshold,after adjusting one or both as needed based on atrial event evidenceand/or A1-A2 interval shortening detection. If the AF evidence score isgreater than the AF threshold, the cardiac rhythm during the AF evidencescore time segment (over which the VCLs were acquired) is classified asAF at block 714. If not, the cardiac rhythm is not classified as AF atblock 716. The process may return to block 702 to determine the next AFevidence score over the next time segment.

FIG. 11 is a graph of example an accelerometer signal 802, ECG signal804 with event markers and ventricular EGM signal 806 during anon-atrial tracking pacing mode with normal AV conduction intact. Theaccelerometer signal 802 is an example of a motion signal that isreceived by the control circuit 206 from the motion sensor 212 (shown inFIG. 3). Accelerometer signal 802 is shown as a non-rectified signal inFIG. 11 but may be rectified by motion sensor 212 or control circuit 206for sensing A4 events. The A1, A2, A3 and A4 events are denoted alongthe motion signal 802.

The “VE” markers shown along ECG signal 804 indicate the end of the A3window, e.g., corresponding to ending time 425 shown in FIG. 7. The “AS”markers indicate the time of an A4 sensed event, e.g., when the A4signals of accelerometer signal 802 cross the A4 sensing threshold. The“VS” markers indicate an R-wave sensed event. R-waves (labeled “R”) ofEGM signal 806 are sensed by sensing circuit 204, which passes an R-wavesensed event signal to control circuit 206. In this example, one A4signal is being sensed per VCL (each RR interval 810). The occurrence ofone A4 signal sensed per VCL is evidence against atrial tachyarrhythmia.In this example VCLs (determined as RR intervals 810) are not irregular,so the AF evidence score would be very low or even negative.

FIG. 12 is a graph of an example accelerometer signal 822, ECG signal824 with event markers, and ventricular EGM signal 826 during atrialfibrillation. No clear A4 signals are observed on the accelerometersignal 822. Following each ending time (VE) of the A3 window, no ASmarker is present indicating that no A4 events are being detected. TheVS markers indicate the time of R-wave sensed events, corresponding toR-waves of the EGM signal 826. The RR intervals 810 in this example areobserved to be irregular. This irregularity is likely to lead to arelatively high AF evidence score. The absence of A4 events results inan atrial event evidence score of zero, such that no adjustments to theAF evidence score (or AF threshold) is made. AF is likely to be properlydetected based on the VCL irregularity and the absence of any A4 eventsensing from the motion signal.

Various examples of a medical device and method for detecting an atrialtachyarrhythmia using an intra-ventricular motion signal have beendescribed. It should be understood that, depending on the example,certain acts or events of any of the methods described herein can beperformed in a different sequence, may be added, merged, or left outaltogether (e.g., not all described acts or events are necessary for thepractice of the method). Moreover, in certain examples, acts or eventsmay be performed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors, rather than sequentially.In addition, while certain aspects of this disclosure are described asbeing performed by a single circuit or unit for purposes of clarity, itshould be understood that the techniques of this disclosure may beperformed by a combination of units or circuits associated with, forexample, a medical device.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored as one or more instructions orcode on a computer-readable medium and executed by a hardware-basedprocessing unit. Computer-readable media may include computer-readablestorage media, which corresponds to a tangible medium such as datastorage media (e.g., RAM, ROM, EEPROM, flash memory, or any other mediumthat can be used to store desired program code in the form ofinstructions or data structures and that can be accessed by a computer).

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablelogic arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto any of the foregoing structure or any other structure suitable forimplementation of the techniques described herein. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

Thus, a medical device has been presented in the foregoing descriptionwith reference to specific examples. It is to be understood that variousaspects disclosed herein may be combined in different combinations thanthe specific combinations presented in the accompanying drawings. It isappreciated that various modifications to the referenced examples may bemade without departing from the scope of the disclosure and thefollowing claims.

What is claimed is:
 1. An implantable medical device, comprising: asensor configured to produce an intraventricular signal that includes anatrial mechanical event signal attendant to atrial mechanical systole; asensing circuit configured to sense ventricular events; a memoryconfigured to store atrial arrhythmia episode data; and a controlcircuit coupled to the sensor, the sensing circuit, and the memory, thecontrol circuit configured to: determine a first atrial arrhythmia scorefrom the ventricular events sensed by the sensing circuit, the firstatrial arrhythmia score correlated to a presence of atrial arrhythmia;determine a second atrial arrhythmia score from the intraventricularsignal based on the atrial mechanical event signals by: identifying aplurality of ventricular events; starting an atrial event windowfollowing each one of the ventricular events; determining an atrialevent window duration from the start of the atrial event window until anext one of the plurality of ventricular events; in response to theatrial event window duration being less than a threshold windowduration, ignore the atrial event window in determining the secondatrial arrhythmia score; determining a count of atrial mechanical eventsignals that are sensed from the intraventricular signal during theatrial event windows that are at least the threshold window duration;and determining the second atrial arrhythmia score based on the count ofsensed atrial mechanical event signals; detect an atrial arrhythmiabased on the first atrial arrhythmia score and the second atrialarrhythmia score; and update the atrial arrhythmia episode data storedin the memory in response to detecting the atrial arrhythmia, the updateincluding an indication of the detected atrial arrhythmia.
 2. Thepacemaker of claim 1, wherein the control circuit is further configuredto detect the atrial arrhythmia by: adjusting the first atrialarrhythmia score based on the second atrial arrhythmia score; comparingthe adjusted first atrial arrhythmia score to an arrhythmia threshold;and detecting the atrial arrhythmia in response to the adjusted firstatrial arrhythmia score being greater than the arrhythmia threshold. 3.The device of claim 2, wherein the control circuit is further configuredto adjust the arrhythmia threshold in response to the second atrialarrhythmia score being less than an adjustment threshold.
 4. The deviceof claim 2, wherein the control circuit is further configured to: detecta plurality of atrial mechanical events from the intraventricular signalduring an atrial event window; and adjust at least one of the firstatrial arrhythmia score and the arrhythmia threshold in response todetecting the plurality of atrial mechanical events during the atrialevent window.
 5. The device of claim 1, wherein the control circuit isfurther configured to: detect a plurality of atrial mechanical eventsfrom the intraventricular signal during an atrial event window; anddetect atrial flutter in response to detecting the plurality of atrialmechanical events.
 6. The device of claim 1, wherein the control circuitis configured to detect the atrial arrhythmia by: comparing the firstatrial arrhythmia score to a first threshold; comparing the secondatrial arrhythmia score to a second threshold; and detecting the atrialarrhythmia in response to the first atrial arrhythmia score beinggreater than the first threshold and the second atrial arrhythmia scorebeing less than the second threshold.
 7. The device of claim 1, whereinthe sensor comprises a motion sensor configured to produce anintraventricular motion signal comprising the atrial mechanical eventsignal.
 8. The device of claim 1, wherein the sensor comprises one of animpedance sensor configured to produce an intraventricular impedancesignal and a pressure sensor configured to produce an intraventricularpressure signal; wherein the control circuit is configured to determinethe second atrial arrhythmia score by sensing atrial systolic eventsfrom one of the intraventricular impedance sensor and theintraventricular pressure sensor.
 9. The device of claim 1, wherein: thesensing circuit is configured to sense R-waves from a cardiac electricalsignal; and the control circuit is configured to: identify the pluralityof ventricular events comprising the R-waves sensed by the sensingcircuit; determine the first atrial arrhythmia score by determining aplurality of ventricular cycle lengths between consecutive pairs of theplurality of ventricular events; and determine the second atrialarrhythmia score from the intraventricular signal during a time segmentcomprising the plurality of ventricular cycle lengths.
 10. The device ofclaim 1, further comprising a pulse generator configured to deliverventricular pacing pulses; wherein the control circuit is furtherconfigured to: identify a plurality of ventricular events comprising theventricular pacing pulses delivered by the pulse generator; determine apercentage of the plurality of ventricular events that are ventricularpacing pulses; and withhold the atrial arrhythmia detection in responseto the percentage being greater than a threshold percentage.
 11. Thedevice of claim 1, wherein the control circuit is further configured to:detect shortening of a ventricular systolic time interval from theintraventricular signal; adjust an arrhythmia threshold in response todetecting the shortening; adjust the first atrial arrhythmia score basedon the second atrial arrhythmia score; and detect the atrial arrhythmiain response to the adjusted first atrial arrhythmia score being greaterthan the adjusted arrhythmia threshold.
 12. The device of claim 1,further comprising a pulse generator configured to deliver ventricularpacing pulses; wherein the control circuit is configured to: determinethe first atrial arrhythmia score by identifying a plurality ofventricular events comprising the delivered ventricular pacing pulsesand determining ventricular cycle length differences from the pluralityof ventricular events; determine the count of atrial mechanical eventsignals by further determining a count of ventricular cycles of theplurality of ventricular events during which a single atrial mechanicalevent signal is sensed from the intraventricular signal; determine apercentage of the identified plurality of ventricular events that areventricular pacing pulses; decrease an atrial arrhythmia detectionthreshold in response to the second atrial arrhythmia score being lessthan an atrial event threshold and the percentage of the identifiedplurality of ventricular events that are ventricular pacing pulses beinggreater than a pacing frequency threshold; and detect the atrialarrhythmia in response to the first atrial arrhythmia score beinggreater than the decreased atrial arrhythmia detection threshold. 13.The device of claim 1, further comprising a pulse generator configuredto: generate ventricular pacing pulses according to a first pacing mode;and in response to the control circuit detecting the atrial arrhythmia,generate the ventricular pacing pulses according to a second pacing modedifferent than the first pacing mode.
 14. The device of claim 1, furthercomprising a pulse generator configured to deliver anti-tachycardiapacing therapy; wherein the control circuit is further configured tocontrol the pulse generator to withhold the anti-tachycardia pacingtherapy in response to detecting the atrial arrhythmia.
 15. The deviceof claim 1, further comprising a telemetry circuit configured totransmit the atrial arrhythmia episode data in response to the updating.16. A method, comprising: determining a first atrial arrhythmia scorefrom ventricular events sensed by a sensing circuit of an implantablemedical device, the first atrial arrhythmia score correlated to apresence of atrial arrhythmia; determining a second atrial arrhythmiascore from an intraventricular signal produced by a sensor of theimplantable medical device, the intraventricular signal comprising anatrial mechanical event signal attendant to atrial systole, whereindetermining the second atrial arrhythmia score comprises: identifying aplurality of ventricular events; starting an atrial event windowfollowing each one of the ventricular events; determining an atrialevent window duration from the start of the atrial event window until anext one of the plurality of ventricular events; in response to theatrial event window duration being less than a threshold windowduration, ignoring the atrial event window in determining the secondatrial arrhythmia score; determining a count of atrial mechanical eventsignals that are sensed from the intraventricular signal during theatrial event windows that are at least the threshold window duration;and determining the second atrial arrhythmia score based on the count ofsensed atrial mechanical event signals; detecting an atrial arrhythmiabased on the first atrial arrhythmia score and the second atrialarrhythmia score; updating atrial arrhythmia episode data stored in amemory of the implantable medical device in response to detecting theatrial arrhythmia, the update including an indication of the detectedatrial arrhythmia.
 17. The method of claim 16, wherein detecting theatrial arrhythmia comprises: adjusting the first atrial arrhythmia scorebased on the second atrial arrhythmia score; comparing the adjustedfirst atrial arrhythmia score to an arrhythmia threshold; detecting theatrial arrhythmia in response to the adjusted first atrial arrhythmiascore being greater than the arrhythmia threshold.
 18. The method ofclaim 17, wherein the control circuit is further configured to adjustthe arrhythmia threshold in response to the second atrial arrhythmiascore being less than an adjustment threshold.
 19. The method of claim17, further comprising: detecting a plurality of atrial mechanicalevents from the intraventricular signal during an atrial event window;and adjusting at least one of the first atrial arrhythmia score and thearrhythmia threshold in response to detecting the plurality of atrialmechanical events during the atrial event window.
 20. The method ofclaim 16, further comprising: detecting a plurality of atrial mechanicalevents from the intraventricular signal during an atrial event window;and detecting atrial flutter in response to detecting the plurality ofatrial mechanical events.
 21. The method of claim 16, wherein detectingthe atrial arrhythmia comprises: comparing the first atrial arrhythmiascore to a first threshold; comparing the second atrial arrhythmia scoreto a second threshold; and detecting the atrial arrhythmia in responseto the first atrial arrhythmia score being greater than the firstthreshold and the second atrial arrhythmia score being less than thesecond threshold.
 22. The method of claim 16, wherein determining thesecond atrial arrhythmia score comprises sensing the atrial mechanicalevent signals from an intraventricular motion signal produced by amotion sensor of the sensor.
 23. The method of claim 16, whereindetermining the second atrial arrhythmia score comprises sensing atrialmechanical events from one of an intraventricular impedance signalproduced by the sensor and an intraventricular pressure signal producedby the sensor.
 24. The method of claim 16, further comprising: sensingR-waves from a cardiac electrical signal received by the sensingcircuit; identifying the plurality of ventricular events comprising theR-waves sensed by the sensing circuit from the cardiac electricalsignal; determining the first atrial arrhythmia score by determining aplurality of ventricular cycle lengths between consecutive pairs of theplurality of ventricular events; and determining the second atrialarrhythmia score from the intraventricular signal during a time segmentcomprising the plurality of ventricular cycle lengths.
 25. The method ofclaim 16, further comprising: identifying a plurality of ventricularevents comprising ventricular pacing pulse pulses delivered by a pulsegenerator of the implantable medical device; determining a percentage ofthe plurality of ventricular events that are ventricular pacing pulses;and withholding the atrial arrhythmia detection in response to thepercentage being greater than a threshold percentage.
 26. The method ofclaim 16, further comprising: detecting shortening of a ventricularsystolic time interval from the intraventricular signal; adjusting anarrhythmia threshold in response to detecting the shortening; adjustingthe first atrial arrhythmia score based on the second atrial arrhythmiascore; and detecting the atrial arrhythmia in response to the adjustedfirst atrial arrhythmia score being greater than the adjusted arrhythmiathreshold.
 27. The method of claim 16, further comprising: determiningthe first atrial arrhythmia score by identifying a plurality ofventricular events comprising ventricular pacing pulses delivered by apulse generator of the implantable medical device and determiningventricular cycle length differences from the plurality of ventricularevents; determining the count of atrial mechanical event signals byfurther determining a count of ventricular cycles of the plurality ofventricular events during which a single atrial mechanical event signalis sensed from the intraventricular signal; determining a percentage ofthe identified plurality of ventricular events that are ventricularpacing pulses; decreasing an atrial arrhythmia detection threshold inresponse to the second atrial arrhythmia score being less than an atrialevent threshold and the percentage of the identified plurality ofventricular events that are ventricular pacing pulses being greater thana pacing frequency threshold; and detect the atrial arrhythmia inresponse to the first atrial arrhythmia score being greater than thedecreased atrial arrhythmia detection threshold.
 28. The method of claim16, further comprising: generating ventricular pacing pulses by a pulsegenerator of the implantable medical device according to a first pacingmode; and in response to detecting the atrial arrhythmia switch togenerating the ventricular pacing pulses by the pulse generatoraccording to a second pacing mode different than the first pacing mode.29. The method of claim 16, further comprising controlling a pulsegenerator of the implantable medical device to withhold ananti-tachycardia pacing therapy in response to detecting the atrialarrhythmia.
 30. The method of claim 16, further comprising transmittingthe atrial arrhythmia episode data by a telemetry circuit of theimplantable medical device.
 31. A non-transitory, computer-readablestorage medium comprising a set of instructions which, when executed bya control circuit of an implantable medical device, cause the device to:determine a first atrial arrhythmia score from ventricular events sensedby a sensing circuit of the implantable medical device, the first atrialarrhythmia score correlated to a presence of atrial arrhythmia;determine a second atrial arrhythmia score from an intraventricularsignal produced by a sensor of the implantable medical device, theintraventricular signal comprising an atrial mechanical event signalattendant to atrial systole, the second atrial arrhythmia score beingdetermined by: identifying a plurality of ventricular events; startingan atrial event window following each one of the ventricular events;determining an atrial event window duration from the start of the atrialevent window until a next one of the plurality of ventricular events; inresponse to the atrial event window duration being less than a thresholdwindow duration, ignoring the atrial event window in determining thesecond atrial arrhythmia score; determining a count of atrial mechanicalevent signals that are sensed from the intraventricular signal duringthe atrial event windows that are at least the threshold windowduration; and determining the second atrial arrhythmia score based onthe count of sensed atrial mechanical event signals; detect an atrialarrhythmia based on the first atrial arrhythmia score and the secondatrial arrhythmia score; update atrial arrhythmia episode data stored ina memory of the implantable medical device in response to detecting theatrial arrhythmia, the update including an indication of the detectedatrial arrhythmia.
 32. An implantable medical device, comprising: asensor configured to produce an intraventricular signal that includes anatrial mechanical event signal attendant to atrial mechanical systole; asensing circuit configured to sense ventricular events; a memoryconfigured to store atrial arrhythmia episode data; and a controlcircuit coupled to the sensor, the sensing circuit, and the memory, thecontrol circuit configured to: determine a first atrial arrhythmia scorefrom the ventricular events sensed by the sensing circuit, the firstatrial arrhythmia score correlated to a presence of atrial arrhythmia;determine a second atrial arrhythmia score from the intraventricularsignal based on the atrial mechanical event signals, wherein the controlcircuit is configured to determine the second atrial arrhythmia scorefrom the intraventricular signal by: sensing a systolic ventricularevent signal from the intraventricular signal; sensing a diastolicventricular event signal from the intraventricular signal; determining aventricular systolic time interval extending from the systolicventricular event signal to the diastolic ventricular event signal;comparing the ventricular systolic time interval to a shorteningthreshold; detecting ventricular systolic shortening based on thecomparison of the ventricular systolic time interval to the shorteningthreshold; and increasing the second atrial arrhythmia score in responseto detecting the ventricular systolic shortening; detect an atrialarrhythmia based on the first atrial arrhythmia score and the secondatrial arrhythmia score; and update the atrial arrhythmia episode datastored in the memory in response to detecting the atrial arrhythmia, theupdate including an indication of the detected atrial arrhythmia.
 33. Amethod, comprising: determining a first atrial arrhythmia score fromventricular events sensed by a sensing circuit of an implantable medicaldevice, the first atrial arrhythmia score correlated to a presence ofatrial arrhythmia; determining a second atrial arrhythmia score from anintraventricular signal produced by a sensor of the implantable medicaldevice, the intraventricular signal comprising an atrial mechanicalevent signal attendant to atrial systole, the second atrial arrhythmiascore based on the atrial mechanical event signals; wherein determiningthe second atrial arrhythmia score from the intraventricular signalcomprises: sensing a systolic ventricular event signal from theintraventricular signal; sensing a diastolic ventricular event signalfrom the intraventricular signal; determining a ventricular systolictime interval extending from the systolic ventricular event signal tothe diastolic ventricular event signal; comparing the ventricularsystolic time interval to a shortening threshold; detecting ventricularsystolic shortening based on the comparison of the ventricular systolictime interval to the shortening threshold; and increasing the secondatrial arrhythmia score in response to detecting the ventricularsystolic shortening; detecting an atrial arrhythmia based on the firstatrial arrhythmia score and the second atrial arrhythmia score; updatingatrial arrhythmia episode data stored in a memory of the implantablemedical device in response to detecting the atrial arrhythmia, theupdate including an indication of the detected atrial arrhythmia.